In the last 15 years, polymer electrolyte membrane (PEM) fuel cells have received much attention, mostly through experimental and empirical studies in scientific and industrial research. In most of the works, attention has been given to the steady state analysis of the PEM fuel cells. However, considerable efforts still need to be done to explain different transient behaviors of PEM fuel cells. This paper presents an analysis of the double layer charging effect and reactant diffusion through the cathode gas diffusion layer on voltage transients after sudden current variations. These transient phenomena have typical time durations of less than 5 s. The double layer charging dynamic explains the main voltage transient behaviors when the cathode inlet pressure is constant. In this case, a bicriteria optimization procedure is proposed for numerical characterization of the double-layer charging capacitance. When the air pressure is variable, a pseudo 2-D modeling of oxygen diffusion through the cathode gas diffusion layers, based on the Stephan - Maxwell multicomponent diffusion equations, is used to explain its contribution to the voltage transient overshoots/undershoots.