diff --git a/src/transaction_builder.js b/src/transaction_builder.js
index ca59d88..51bc396 100644
--- a/src/transaction_builder.js
+++ b/src/transaction_builder.js
@@ -468,14 +468,14 @@ function buildInput (input, allowIncomplete) {
 }
 
 function TransactionBuilder (network, maximumFeeRate) {
-  this.prevTxMap = {}
+  this.__prevTxSet = {}
   this.network = network || networks.bitcoin
 
   // WARNING: This is __NOT__ to be relied on, its just another potential safety mechanism (safety in-depth)
   this.maximumFeeRate = maximumFeeRate || 2500
 
-  this.inputs = []
-  this.tx = new Transaction()
+  this.__inputs = []
+  this.__tx = new Transaction()
   this.tx.version = 2
 }
 
@@ -483,7 +483,7 @@ TransactionBuilder.prototype.setLockTime = function (locktime) {
   typeforce(types.UInt32, locktime)
 
   // if any signatures exist, throw
-  if (this.inputs.some(function (input) {
+  if (this.__inputs.some(function (input) {
     if (!input.signatures) return false
 
     return input.signatures.some(function (s) { return s })
@@ -491,14 +491,14 @@ TransactionBuilder.prototype.setLockTime = function (locktime) {
     throw new Error('No, this would invalidate signatures')
   }
 
-  this.tx.locktime = locktime
+  this.__tx.locktime = locktime
 }
 
 TransactionBuilder.prototype.setVersion = function (version) {
   typeforce(types.UInt32, version)
 
   // XXX: this might eventually become more complex depending on what the versions represent
-  this.tx.version = version
+  this.__tx.version = version
 }
 
 TransactionBuilder.fromTransaction = function (transaction, network) {
@@ -523,7 +523,7 @@ TransactionBuilder.fromTransaction = function (transaction, network) {
   })
 
   // fix some things not possible through the public API
-  txb.inputs.forEach(function (input, i) {
+  txb.__inputs.forEach(function (input, i) {
     fixMultisigOrder(input, transaction, i)
   })
 
@@ -564,7 +564,7 @@ TransactionBuilder.prototype.__addInputUnsafe = function (txHash, vout, options)
   }
 
   var prevTxOut = txHash.toString('hex') + ':' + vout
-  if (this.prevTxMap[prevTxOut] !== undefined) throw new Error('Duplicate TxOut: ' + prevTxOut)
+  if (this.__prevTxSet[prevTxOut] !== undefined) throw new Error('Duplicate TxOut: ' + prevTxOut)
 
   var input = {}
 
@@ -597,9 +597,9 @@ TransactionBuilder.prototype.__addInputUnsafe = function (txHash, vout, options)
     input.prevOutType = prevOutType || btemplates.classifyOutput(options.prevOutScript)
   }
 
-  var vin = this.tx.addInput(txHash, vout, options.sequence, options.scriptSig)
-  this.inputs[vin] = input
-  this.prevTxMap[prevTxOut] = vin
+  var vin = this.__tx.addInput(txHash, vout, options.sequence, options.scriptSig)
+  this.__inputs[vin] = input
+  this.__prevTxSet[prevTxOut] = true
   return vin
 }
 
@@ -613,7 +613,7 @@ TransactionBuilder.prototype.addOutput = function (scriptPubKey, value) {
     scriptPubKey = baddress.toOutputScript(scriptPubKey, this.network)
   }
 
-  return this.tx.addOutput(scriptPubKey, value)
+  return this.__tx.addOutput(scriptPubKey, value)
 }
 
 TransactionBuilder.prototype.build = function () {
@@ -625,13 +625,13 @@ TransactionBuilder.prototype.buildIncomplete = function () {
 
 TransactionBuilder.prototype.__build = function (allowIncomplete) {
   if (!allowIncomplete) {
-    if (!this.tx.ins.length) throw new Error('Transaction has no inputs')
-    if (!this.tx.outs.length) throw new Error('Transaction has no outputs')
+    if (!this.__tx.ins.length) throw new Error('Transaction has no inputs')
+    if (!this.__tx.outs.length) throw new Error('Transaction has no outputs')
   }
 
-  var tx = this.tx.clone()
+  var tx = this.__tx.clone()
   // Create script signatures from inputs
-  this.inputs.forEach(function (input, i) {
+  this.__inputs.forEach(function (input, i) {
     var scriptType = input.witnessScriptType || input.redeemScriptType || input.prevOutType
     if (!scriptType && !allowIncomplete) throw new Error('Transaction is not complete')
     var result = buildInput(input, allowIncomplete)
@@ -673,10 +673,10 @@ function canSign (input) {
 TransactionBuilder.prototype.sign = function (vin, keyPair, redeemScript, hashType, witnessValue, witnessScript) {
   // TODO: remove keyPair.network matching in 4.0.0
   if (keyPair.network && keyPair.network !== this.network) throw new TypeError('Inconsistent network')
-  if (!this.inputs[vin]) throw new Error('No input at index: ' + vin)
+  if (!this.__inputs[vin]) throw new Error('No input at index: ' + vin)
   hashType = hashType || Transaction.SIGHASH_ALL
 
-  var input = this.inputs[vin]
+  var input = this.__inputs[vin]
 
   // if redeemScript was previously provided, enforce consistency
   if (input.redeemScript !== undefined &&
@@ -700,9 +700,9 @@ TransactionBuilder.prototype.sign = function (vin, keyPair, redeemScript, hashTy
   // ready to sign
   var signatureHash
   if (input.witness) {
-    signatureHash = this.tx.hashForWitnessV0(vin, input.signScript, input.value, hashType)
+    signatureHash = this.__tx.hashForWitnessV0(vin, input.signScript, input.value, hashType)
   } else {
-    signatureHash = this.tx.hashForSignature(vin, input.signScript, hashType)
+    signatureHash = this.__tx.hashForSignature(vin, input.signScript, hashType)
   }
 
   // enforce in order signing of public keys
@@ -731,7 +731,7 @@ function signatureHashType (buffer) {
 }
 
 TransactionBuilder.prototype.__canModifyInputs = function () {
-  return this.inputs.every(function (input) {
+  return this.__inputs.every(function (input) {
     // any signatures?
     if (input.signatures === undefined) return true
 
@@ -747,10 +747,10 @@ TransactionBuilder.prototype.__canModifyInputs = function () {
 }
 
 TransactionBuilder.prototype.__canModifyOutputs = function () {
-  var nInputs = this.tx.ins.length
-  var nOutputs = this.tx.outs.length
+  var nInputs = this.__tx.ins.length
+  var nOutputs = this.__tx.outs.length
 
-  return this.inputs.every(function (input) {
+  return this.__inputs.every(function (input) {
     if (input.signatures === undefined) return true
 
     return input.signatures.every(function (signature) {
@@ -771,11 +771,11 @@ TransactionBuilder.prototype.__canModifyOutputs = function () {
 
 TransactionBuilder.prototype.__overMaximumFees = function (bytes) {
   // not all inputs will have .value defined
-  var incoming = this.inputs.reduce(function (a, x) { return a + (x.value >>> 0) }, 0)
+  var incoming = this.__inputs.reduce(function (a, x) { return a + (x.value >>> 0) }, 0)
 
   // but all outputs do, and if we have any input value
   // we can immediately determine if the outputs are too small
-  var outgoing = this.tx.outs.reduce(function (a, x) { return a + x.value }, 0)
+  var outgoing = this.__tx.outs.reduce(function (a, x) { return a + x.value }, 0)
   var fee = incoming - outgoing
   var feeRate = fee / bytes
 
diff --git a/test/transaction_builder.js b/test/transaction_builder.js
index 35da882..cc24571 100644
--- a/test/transaction_builder.js
+++ b/test/transaction_builder.js
@@ -133,10 +133,11 @@ describe('TransactionBuilder', function () {
       })
     })
 
-    it('correctly classifies transaction inputs', function () {
+    it('classifies transaction inputs', function () {
       var tx = Transaction.fromHex(fixtures.valid.classification.hex)
       var txb = TransactionBuilder.fromTransaction(tx)
-      txb.inputs.forEach(function (i) {
+
+      txb.__inputs.forEach(function (i) {
         assert.strictEqual(i.prevOutType, 'scripthash')
         assert.strictEqual(i.redeemScriptType, 'multisig')
         assert.strictEqual(i.signType, 'multisig')
@@ -164,22 +165,22 @@ describe('TransactionBuilder', function () {
       var vin = txb.addInput(txHash, 1, 54)
       assert.strictEqual(vin, 0)
 
-      var txIn = txb.tx.ins[0]
+      var txIn = txb.__tx.ins[0]
       assert.strictEqual(txIn.hash, txHash)
       assert.strictEqual(txIn.index, 1)
       assert.strictEqual(txIn.sequence, 54)
-      assert.strictEqual(txb.inputs[0].prevOutScript, undefined)
+      assert.strictEqual(txb.__inputs[0].prevOutScript, undefined)
     })
 
     it('accepts a txHash, index [, sequence number and scriptPubKey]', function () {
       var vin = txb.addInput(txHash, 1, 54, scripts[1])
       assert.strictEqual(vin, 0)
 
-      var txIn = txb.tx.ins[0]
+      var txIn = txb.__tx.ins[0]
       assert.strictEqual(txIn.hash, txHash)
       assert.strictEqual(txIn.index, 1)
       assert.strictEqual(txIn.sequence, 54)
-      assert.strictEqual(txb.inputs[0].prevOutScript, scripts[1])
+      assert.strictEqual(txb.__inputs[0].prevOutScript, scripts[1])
     })
 
     it('accepts a prevTx, index [and sequence number]', function () {
@@ -190,11 +191,11 @@ describe('TransactionBuilder', function () {
       var vin = txb.addInput(prevTx, 1, 54)
       assert.strictEqual(vin, 0)
 
-      var txIn = txb.tx.ins[0]
+      var txIn = txb.__tx.ins[0]
       assert.deepEqual(txIn.hash, prevTx.getHash())
       assert.strictEqual(txIn.index, 1)
       assert.strictEqual(txIn.sequence, 54)
-      assert.strictEqual(txb.inputs[0].prevOutScript, scripts[1])
+      assert.strictEqual(txb.__inputs[0].prevOutScript, scripts[1])
     })
 
     it('returns the input index', function () {
@@ -222,7 +223,7 @@ describe('TransactionBuilder', function () {
       var vout = txb.addOutput(keyPair.getAddress(), 1000)
       assert.strictEqual(vout, 0)
 
-      var txout = txb.tx.outs[0]
+      var txout = txb.__tx.outs[0]
       assert.deepEqual(txout.script, scripts[0])
       assert.strictEqual(txout.value, 1000)
     })
@@ -231,7 +232,7 @@ describe('TransactionBuilder', function () {
       var vout = txb.addOutput(scripts[0], 1000)
       assert.strictEqual(vout, 0)
 
-      var txout = txb.tx.outs[0]
+      var txout = txb.__tx.outs[0]
       assert.deepEqual(txout.script, scripts[0])
       assert.strictEqual(txout.value, 1000)
     })
@@ -505,10 +506,10 @@ describe('TransactionBuilder', function () {
       '194a565cd6aa4cc38b8eaffa343402201c5b4b61d73fa38e49c1ee68cc0e6dfd2f5dae453dd86eb142e87a' +
       '0bafb1bc8401210283409659355b6d1cc3c32decd5d561abaac86c37a353b52895a5e6c196d6f44800000000'
       var txb = TransactionBuilder.fromTransaction(Transaction.fromHex(rawtx))
-      txb.inputs[0].value = 241530
-      txb.inputs[1].value = 241530
-      txb.inputs[2].value = 248920
-      txb.inputs[3].value = 248920
+      txb.__inputs[0].value = 241530
+      txb.__inputs[1].value = 241530
+      txb.__inputs[2].value = 248920
+      txb.__inputs[3].value = 248920
 
       assert.throws(function () {
         txb.build()
@@ -532,7 +533,7 @@ describe('TransactionBuilder', function () {
       var txHex = tx.toHex()
       var newTxb = TransactionBuilder.fromTransaction(Transaction.fromHex(txHex))
       // input should have the key 'witness' set to true
-      assert.equal(newTxb.inputs[0].witness, true)
+      assert.equal(newTxb.__inputs[0].witness, true)
     })
 
     it('should handle badly pre-filled OP_0s', function () {