In an apparatus with transactional memory support circuitry, for a first type of transaction started using a first type of transaction start instruction, commitment of results of instructions executed speculatively following the first type of transaction start instruction are prevented until a transaction end instruction is reached. An abort is triggered when a conflict is detected between an address of a memory access from another thread and the addresses tracked for the transaction. For a second type of transaction started using a second type of transaction start instruction, an address of the read operation is marked as trackable whilst an address of a write operation is omitted from being marked as trackable. This allows an apparatus that supports transactional memory to also be used for multi-word address watching.