diff --git a/libethereum/ClientBase.cpp b/libethereum/ClientBase.cpp
index 3d4411505..d6d259380 100644
--- a/libethereum/ClientBase.cpp
+++ b/libethereum/ClientBase.cpp
@@ -481,7 +481,7 @@ int ClientBase::compareBlockHashes(h256 _h1, h256 _h2) const
 	return -1;
 }
 
-bool ClientBase::isKnown(h256 _hash) const
+bool ClientBase::isKnown(h256 const& _hash) const
 {
 	return _hash == PendingBlockHash ||
 		_hash == LatestBlockHash ||
@@ -496,7 +496,7 @@ bool ClientBase::isKnown(BlockNumber _block) const
 		bc().numberHash(_block) != h256();
 }
 
-bool ClientBase::isKnownTransaction(h256 _transactionHash) const
+bool ClientBase::isKnownTransaction(h256 const& _transactionHash) const
 {
 	return bc().isKnownTransaction(_transactionHash);
 }
diff --git a/libethereum/ClientBase.h b/libethereum/ClientBase.h
index aac10ae60..d2d22a08f 100644
--- a/libethereum/ClientBase.h
+++ b/libethereum/ClientBase.h
@@ -149,9 +149,9 @@ public:
 	/// Get the coinbase address
 	virtual Address address() const override;
 
-	virtual bool isKnown(h256 _hash) const override;
+	virtual bool isKnown(h256 const& _hash) const override;
 	virtual bool isKnown(BlockNumber _block) const override;
-	virtual bool isKnownTransaction(h256 _transactionHash) const override;
+	virtual bool isKnownTransaction(h256 const& _transactionHash) const override;
 
 	/// TODO: consider moving it to a separate interface
 
diff --git a/libethereum/Interface.h b/libethereum/Interface.h
index 1073b45ff..f24f1b5c3 100644
--- a/libethereum/Interface.h
+++ b/libethereum/Interface.h
@@ -133,7 +133,7 @@ public:
 
 	// [BLOCK QUERY API]
 
-	virtual bool isKnownTransaction(h256 _transactionHash) const = 0;
+	virtual bool isKnownTransaction(h256 const& _transactionHash) const = 0;
 	virtual Transaction transaction(h256 _transactionHash) const = 0;
 	virtual std::pair<h256, unsigned> transactionLocation(h256 const& _transactionHash) const = 0;
 	virtual h256 hashFromNumber(BlockNumber _number) const = 0;
@@ -141,7 +141,7 @@ public:
 	virtual int compareBlockHashes(h256 _h1, h256 _h2) const = 0;
 
 	virtual bool isKnown(BlockNumber _block) const = 0;
-	virtual bool isKnown(h256 _hash) const = 0;
+	virtual bool isKnown(h256 const& _hash) const = 0;
 	virtual BlockInfo blockInfo(h256 _hash) const = 0;
 	virtual BlockDetails blockDetails(h256 _hash) const = 0;
 	virtual Transaction transaction(h256 _blockHash, unsigned _i) const = 0;