The aim of this paper is to present a simple 3D computational model of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) that simulates over time the heat distribution, energy and mass balance of the reactant gas flows in the fuel cell including pressure drop, humidity and liquid water. Although this theoretical model can be adapted to any type of PEMFC, for verification of the model and to present different analysis, it has been adapted to a single cell test fixture. The model parameters were adjusted through a series of experimental tests and the model was experimentally validated for a well defined range of operating conditions: H2/Air as reactants, flow rates of 0.5-1.5 SLPM, dew points and cell temperatures of 30-80ºC, currents 0-5 A and with/without water condensation. The model is especially suited for the analysis of liquid water condensation in the reactant channels. A key finding is that the critical current at which liquid water is formed is determined at different flows, temperatures and humidity.