Ontario Hydro has developed a computer program called FLOSHEET for simulation of hydrogen isotope and fusion fuel processing systems. The development of the program was initiated to provide support for the commissioning and operation of the Darlington Tritium Removal Facility (DTRF) at Ontario Hydro's Nuclear Generating Station A. The program has also proven useful in simulating tritium systems in fusion applications. FLOSHEET has been developed as a general purpose simulator which can be used to simulate many different types of processes and process flowsheets. This paper describes the FLOSHEET process simulator and its application to the DTRF and fusion reactor tritium systems.