This investigation on proton exchange membrane fuel cells (PEMFCs) consists of two parts : (i) an experimental analysis of the oxygen reduction reaction, using an O2/He, O2/Ar, or O2/N2 gas mixture as the cathodic reactant, to determine the effects of oxygen partial pressure, temperature, and pressure on the contributions to activation and mass-transport overpotentials; and (ii) a theoretical analysis of oxygen reduction to : first, interpret the experimental results with O2/He, O2/Ar, and O2/N2 gas mixtures as the cathodic reactant, and, second, elucidate the effects of the structure of the electrode and of the physicochemical parameters of the reactant gas, as well as of temperature and pressure, on the oxygen electrode potential vs. current density behavior. For the experimental analysis, which is presented in this Part of the paper, the performance evaluation of PEMFCs (in single cells with the geometric area of the electrodes being 50 cm2) was carried out using O2/He, O2/Ar, and O2/N2 gas mixtures as the cathodic reactants. Experiments were also carried out using air and oxygen. Cell potential vs. current density measurements were made over the temperature range of 50 to 90-degrees-C and pressure range of 1 to 5 atm. These studies showed that mass-transfer limitations are less in PEMFCs with O2/He, than with O2/Ar or O2/N2 gas mixtures. Furthermore, at an oxygen composition greater than 40 % of O2 in the gas mixture, the pseudo-linear region of the potential vs. current density plot is extended at least up to 1 A/cm2. With air as the cathodic reactant, mass-transport effects are minimal at pressures of 3 atm and higher. Temperature effects are less significant than pressure effects, because in the former case there is a compensating effect of higher temperature and lower partial pressure of oxygen on the electrode kinetics of oxygen reduction.