This paper presents a computational investigation of the effect of time-varying modulating conditions on a polymer electrolyte membrane fuel cell. The focus is on developing a better understanding of the fuel cell's water balance under transient conditions, which is critical in improving the fuel cell design. The study employs a macroscopic single-fuel cell-based, one-dimensional, isothermal model. The model 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 fuel cell voltage. The results show that cell voltage 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. The peak cell response is observed at low frequencies. Copyright (c) 2011 John Wiley & Sons, Ltd.