A software transactional memory system is provided that generates and stores compressed transactional locks in a portion of object headers. The software transactional memory system allocates preferred write log memory with a predefined size of memory that corresponds to a number of bits in the compressed transactional locks. The compressed transactional locks identify write log entries in corresponding write logs in the preferred write log memory. If the preferred write log memory becomes full, additional write log memory is allocated for write log entries and subsequent transactional locks are stored uncompressed in an auxiliary memory. A pointer that may be used to locate the uncompressed transactional lock is stored in the header. If an object header with a compressed transactional lock is needed for another use, the compressed transactional lock is uncompressed and stored in the auxiliary memory. A pointer that may be used to locate the uncompressed transactional lock is stored in the header.