This paper presents the development and application of a performance model for a solid polymer fuel cell power system for transportation applications. The model simulates the operation of all the components in the system-including the fuel cell stack, and the air compression, hydrogen supply, and cooling subsystems. Two duty cycles are used to examine the performance of the baseline system, as well as to evaluate the benefits obtained by improving individual system components. The results indicate that the largest improvements in system performance would be obtained by (a) a reduction of the cooling loads, (b) improving the efficiency of the inlet air compressor, and (c) recovery of the thermomechanical energy in the stored compressed hydrogen.