This paper presents a computational investigation of the effect of time-varying humidity conditions on a polymer electrolyte membrane fuel cell. The objective is to develop a better fundamental understanding of the fuel cell's performance under actual driving conditions. Such an understanding will be beneficial in improving the fuel cell design for mobile applications. The study employs a macroscopic single-fuel cell-based, one-dimensional, isothermal model. The novelty of the model is that it does not rely on the non-physical assumption of the uptake curve equilibrium between the pore vapor and ionomer water in the catalyst layers. Instead, the transition between the two phases is modeled as a finite-rate equilibration process. The modulating conditions are simulated by forcing the temporal variations in reactant humidity. The results show that reactant humidity modulations cause a departure in the cell behavior from its steady behavior, and the finite-rate equilibration between the catalyst vapor and liquid water can be a factor in determining the cell response. The cell response is also affected by the modulating frequency and amplitude. Copyright © 2012 John Wiley & Sons, Ltd.