The objective of this study is to investigate the transient behavior of a PEM fuel cell by using a one-dimensional, two-phase mathematical model. This model treats the catalyst layer as a spherical thin film-agglomerate. Effects of various transport parameter as well as other factors such as catalyst loading, gas diffusion layer thickness and liquid water permeability on the transient evolution of major model properties and cell performance are investigated in detail. Numerical results show that the evolution of ionic potential drop and oxygen consumption experience several steps before they reach steady state. The same situation can also be seen for the evolution of water saturation and current density. A detail inspection of these phenomena shows a close relation between these transport variables and cell performance. Parametric studies of other design factors' effects reveal there optimum values which lead to a greater current output during its evolution period. (c) 2007 Elsevier B.V. All rights reserved.