In an apparatus (2) with transactional memory support, a predetermined type of transaction start instruction or a subsequent instruction following the predetermined type of transaction start instruction triggers capture of a lock identifier which identifies a lock variable for controlling exclusive access to at least one resource. In response to a predetermined type of transaction end instruction which follows the predetermined type of transaction start instruction, the lock variable is checked and commitment of results of speculatively executed instructions of the transaction is prevented or deferred when the lock variable indicates that another thread holds the exclusive access to the target resource. This approach can improve performance when executing transactions in a transactional memory based system.