Unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell (FC) and of an electrolyzer (Ely) in a single device. In polymer electrolyte URFCs, gas purging is crucial in switching from the Ely to the FC operation. A gas purge model was developed here to clarify the relationship between the purge time and the gas flow rate. The model considers evaporation and diffusion of liquid water in the gas diffusion layer and membrane. To validate our model, we compared model predictions of high frequency resistance vs purge time with experimental results for two URFCs of different sizes at different purge flow rates. The simulated curves agree well with the experimental curves. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428709] All rights reserved.