diff --git a/alethzero/MainWin.cpp b/alethzero/MainWin.cpp
index 39bc0ad43..9ee3f36e3 100644
--- a/alethzero/MainWin.cpp
+++ b/alethzero/MainWin.cpp
@@ -1629,7 +1629,7 @@ void Main::on_net_triggered()
 	{
 		web3()->setIdealPeerCount(ui->idealPeers->value());
 		web3()->setNetworkPreferences(netPrefs());
-		ethereum()->setNetworkId(m_privateChain.size() ? sha3(m_privateChain.toStdString()) : 0);
+		ethereum()->setNetworkId(m_privateChain.size() ? sha3(m_privateChain.toStdString()) : h256());
 		// TODO: p2p
 //		if (m_networkConfig.size()/* && ui->usePast->isChecked()*/)
 //			web3()->restoreNetwork(bytesConstRef((byte*)m_networkConfig.data(), m_networkConfig.size()));
diff --git a/libdevcore/FixedHash.h b/libdevcore/FixedHash.h
index 561f2f405..2b4e6bc08 100644
--- a/libdevcore/FixedHash.h
+++ b/libdevcore/FixedHash.h
@@ -80,7 +80,7 @@ public:
 	operator Arith() const { return fromBigEndian<Arith>(m_data); }
 
 	/// @returns true iff this is the empty hash.
-	operator bool() const { return ((Arith)*this) != 0; }
+	explicit operator bool() const { return ((Arith)*this) != 0; }
 
 	// The obvious comparison operators.
 	bool operator==(FixedHash const& _c) const { return m_data == _c.m_data; }
diff --git a/libdevcrypto/CryptoPP.cpp b/libdevcrypto/CryptoPP.cpp
index 2d0170fb9..43993e0c5 100644
--- a/libdevcrypto/CryptoPP.cpp
+++ b/libdevcrypto/CryptoPP.cpp
@@ -134,7 +134,7 @@ bool Secp256k1::verify(Signature const& _signature, bytesConstRef _message)
 bool Secp256k1::verify(Public const& _p, Signature const& _sig, bytesConstRef _message, bool _hashed)
 {
 	// todo: verify w/o recovery (if faster)
-	return _p == _hashed ? recover(_sig, _message) : recover(_sig, sha3(_message).ref());
+	return (bool)_p == _hashed ? (bool)recover(_sig, _message) : (bool)recover(_sig, sha3(_message).ref());
 }
 
 Public Secp256k1::recover(Signature _signature, bytesConstRef _message)
diff --git a/libethcore/ProofOfWork.h b/libethcore/ProofOfWork.h
index 64ce502af..c3c3f192b 100644
--- a/libethcore/ProofOfWork.h
+++ b/libethcore/ProofOfWork.h
@@ -82,7 +82,7 @@ template <class Evaluator>
 std::pair<MineInfo, h256> ProofOfWorkEngine<Evaluator>::mine(h256 const& _root, u256 const& _difficulty, unsigned _msTimeout, bool _continue, bool _turbo)
 {
 	std::pair<MineInfo, h256> ret;
-	static std::mt19937_64 s_eng((time(0) + (unsigned)m_last));
+	static std::mt19937_64 s_eng((time(0) + *reinterpret_cast<unsigned*>(m_last.data())));
 	u256 s = (m_last = h256::random(s_eng));
 
 	bigint d = (bigint(1) << 256) / _difficulty;
diff --git a/libethereum/EthereumHost.h b/libethereum/EthereumHost.h
index c732f2dc1..dfa928675 100644
--- a/libethereum/EthereumHost.h
+++ b/libethereum/EthereumHost.h
@@ -94,7 +94,7 @@ private:
 	h256Set neededBlocks(h256Set const& _exclude);
 
 	///	Check to see if the network peer-state initialisation has happened.
-	bool isInitialised() const { return m_latestBlockSent; }
+	bool isInitialised() const { return (bool)m_latestBlockSent; }
 
 	/// Initialises the network peer-state, doing the stuff that needs to be once-only. @returns true if it really was first.
 	bool ensureInitialised();
diff --git a/libethereum/Transaction.h b/libethereum/Transaction.h
index 0f88a4bc0..a9044970f 100644
--- a/libethereum/Transaction.h
+++ b/libethereum/Transaction.h
@@ -81,7 +81,7 @@ public:
 	Address safeSender() const noexcept;
 
 	/// @returns true if transaction is non-null.
-	operator bool() const { return m_type != NullTransaction; }
+	explicit operator bool() const { return m_type != NullTransaction; }
 
 	/// @returns true if transaction is contract-creation.
 	bool isCreation() const { return m_type == ContractCreation; }
diff --git a/libweb3jsonrpc/WebThreeStubServerBase.cpp b/libweb3jsonrpc/WebThreeStubServerBase.cpp
index c304e3c03..dbf3b2ec9 100644
--- a/libweb3jsonrpc/WebThreeStubServerBase.cpp
+++ b/libweb3jsonrpc/WebThreeStubServerBase.cpp
@@ -704,7 +704,7 @@ std::string WebThreeStubServerBase::eth_transact(Json::Value const& _json)
 	if (!t.from)
 		t.from = m_accounts->getDefaultTransactAccount();
 	if (t.creation)
-		ret = right160(sha3(rlpList(t.from, client()->countAt(t.from))));;
+		ret = toJS(right160(sha3(rlpList(t.from, client()->countAt(t.from)))));;
 	if (!t.gasPrice)
 		t.gasPrice = 10 * dev::eth::szabo;
 	if (!t.gas)
diff --git a/libwhisper/Common.cpp b/libwhisper/Common.cpp
index f17ad638b..c29ac6bf6 100644
--- a/libwhisper/Common.cpp
+++ b/libwhisper/Common.cpp
@@ -71,7 +71,7 @@ bool TopicFilter::matches(Envelope const& _e) const
 		for (unsigned i = 0; i < t.size(); ++i)
 		{
 			for (auto et: _e.topic())
-				if (((t[i].first ^ et) & t[i].second) == 0)
+				if (((t[i].first ^ et) & t[i].second) == CollapsedTopicPart())
 					goto NEXT_TOPICPART;
 			// failed to match topicmask against any topics: move on to next mask
 			goto NEXT_TOPICMASK;
diff --git a/mix/ClientModel.cpp b/mix/ClientModel.cpp
index 45198c114..110bdc141 100644
--- a/mix/ClientModel.cpp
+++ b/mix/ClientModel.cpp
@@ -370,7 +370,7 @@ void ClientModel::onNewTransaction()
 	QString function;
 	QString returned;
 
-	bool creation = tr.contractAddress != 0;
+	bool creation = (bool)tr.contractAddress;
 
 	//TODO: handle value transfer
 	FixedHash<4> functionHash;
@@ -403,7 +403,7 @@ void ClientModel::onNewTransaction()
 	if (creation)
 		returned = QString::fromStdString(toJS(tr.contractAddress));
 
-	Address contractAddress = tr.address != 0 ? tr.address : tr.contractAddress;
+	Address contractAddress = (bool)tr.address ? tr.address : tr.contractAddress;
 	auto contractAddressIter = m_contractNames.find(contractAddress);
 	if (contractAddressIter != m_contractNames.end())
 	{