We describe the MPI/SX implementation of the MPI-2 standard for one-sided communication (Remote Memory Access) for the NEC SX-5 vector supercomputer. MPI/SX is a non-threaded implementation of the full MPI-2 standard. Essential features of the implementation are presented, including the synchronization mechanisms, the handling of communication windows in global shared and in process local memory, as well as the handling of MPI derived datatypes. In comparative benchmarks the data transfer operations for one-sided communication and point-to-point message passing show very similar performance, both when data reside in global shared and when in process local memory. Derived datatypes, which are of particular importance for applications using one-sided communications, impose only a modest overhead and can be used without any significant loss of performance. Thus, the MPI/SX programmer can freely choose either the message passing or the one-sided communication model, whichever is most convenient for the given application.