The employment of H-2 in fuel cell (FCs) technologies could ensure significant advantages in terms of efficiency and environmental impact, representing thus an important alternative to the conventional energy production systems. As far as the actual lack of infrastructure for H-2 production, storage, and distribution is concerned, FCs fed with H-2, produced by fossil fuel reforming to generate on-board vehicles at least auxiliary power, represent a valid and interesting alternative to overcome the actual unfavorable situation, waiting for further development of infrastructures. The present work deals with the analysis of a 5 kW(e) auxiliary power unit (APU) integrated with a fuel processor unit (FPU), based on oxidative steam reforming (OSR, with bleed air, BA) and related CO clean-up technologies, for the production of H-2-rich fuel gas, which is fed to a PEM-FC stack. Using the results of a series of steady-state system simulations with Matlab/Simulink, the APU performance was analyzed in terms of FP and APU efficiency, specific power produced by the stack, and water balance, by considering different liquid hydrocarbon fuels (gasoline, light and heavy diesel) and compressed natural gas (CNG) as feed. A parametric sensitivity analysis concerning the effects of the variation of steam to carbon ratio (SCR) and the BA rate to OSR was also carried out. As the main obtained results, the APU performance in transforming the primary fuel in H-2-rich gas was more efficient according to the higher H-2 to carbon ratio HCR of the feedstock. In fact, the APU net efficiency decreased with the following order: methane (27.8%) > gasoline (27.0%) > light diesel (26.5%) > heavy diesel (26.0%). The parametric sensitivity analysis revealed that with small SCR decrease and bleed air rate increase it was possible to improve the APU performance.