A dynamic model of a 1.2 kW polymer electrolyte membrane (PEM) fuel cell (FC) is developed and validated through a series of experiments. This dynamic model is mostly oriented towards control and operation optimization and can be a useful tool for the design of FC-based systems. In the methodology proposed, theoretical equations are combined with experimental relations, resulting in a semi-empirical formulation. The model assumptions are discussed extensively as the equations are presented. This model contributes to the description of the following areas: fluid dynamics in the gas flow fields and gas diffusion layers (oxygen, hydrogen, liquid water and vapor); thermal dynamics and temperature effects; a novel algorithm to calculate an empirical polarization curve. As a result, this model can predict both steady and transient states (such as flooding and anode purges) due to variable loads, as well as the system start-up. Based on this model, a simulator software package has been developed, which is available upon request. The model parameters have been adjusted specifically for a 1.2 kW Ballard stack, which can be considered a benchmark as it is widely used by research groups worldwide. Finally, the simulated results are compared to experimental data from the Ballard stack, demonstrating the accuracy of the proposed model methodology. (C) 2007 Elsevier B.V. All rights reserved.