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 © 2011 John Wiley & Sons, Ltd.