An unsteady two phase nonisothermal model is used to study the start-up characteristics of a polymer electrolyte membrane fuel cell (PEMFC). The model is 3D in nature and a PEMFC with serpentine flow field and 9 channels is simulated. The base case is run with equilibrium or inflow initial conditions, i.e. the initial conditions inside the fuel cell are set to be equal to inflow conditions. The change in current density, at different potentials, with time is plotted. The effect of different parameters on the transient current density behavior is studied. Single phase and two phase models are compared to understand the effect of liquid water on the performance at start-up and is seen to affect the current undershoot Isothermal and nonisothermal models are also compared to see the effect of temperature. Following this, the gas diffusion layer thickness and the porosity are varied and are seen to have a significant effect on the current density. While both have an immediate effect on the mass transport time scales, the current density is affected by both transport and liquid water formation. The time scales of the above two mentioned phenomenon determine how the current behaves at start-up. (C) 2009 Elsevier Ltd. All rights reserved.