Strong semantics are provided to programs that are correctly synchronized in their use of transactions by using dynamic separation of objects that are accessed in transactions from those accessed outside transactions. At run-time, operations are performed to identify transitions between these protected and unprotected modes of access. Dynamic separation permits a range of hardware-based and software-based implementations which allow non-conflicting transactions to execute and commit in parallel. A run-time checking tool, analogous to a data-race detector, may be provided to test dynamic separation of transacted data and non-transacted data. Dynamic separation may be used in an asynchronous I/O library.