An effort is underway to develop a Proton Exchange Membrane (PEM) Fuel Cell system for future space applications. A breadboard system has been constructed and includes the following: reactant pressure regulators, ejector-based reactant pumps, a 4-kW fuel cell stack, and a passive, membrane-based oxygen / water separator. A computer model is being developed concurrently to analytically predict fluid flow in the oxidant reactant system. Preliminary data obtained from tests performed with the breadboard system, as well as other published data, has been used to validate the analytical component models. These components have been incorporated into an integrated oxidant fluid system model. Results obtained from both the performance tests and the analytical model are presented.