The do and ac responses of an active layer in a gas diffusion electrode (GDE) have been predicted from theory, considering both diffusion and ionic ohmic drop inside the active layer and a multistep mechanism. This complete approach required numerical resolution. One-step, two-step (Volmer-Heyrovsky) and three-step mechanisms have been investigated. Numerical simulations from original models using dimensionless parameters lead to impedance diagrams involving a nearly perfect semicircle at low frequency and a 45 angle straight line at high frequency, so proving, respectively, the influence of mass and charge transport processes. Finally, a three-step oxygen reduction mechanism gives rise to theoretical impedance diagrams similar to the experimental ones.