The mobility of a water droplet under the influence of the oxidant fluid in a channel of a PEM fuel cell is studied computationally. The performance of a PEM fuel cell is highly dependent on the oxygen transport rate, which in turn is strongly affected by the presence of liquid water. Excessive liquid water in the cathode causes cathode flooding which is generally known to be the primary reason for low cell performance. The exerted forces by the oxidant flow help to remove most of the liquid water that is entrapped in the porous electrode, hence minimizing electrode flooding. In this study the effects of liquid water and surrounding fluid properties on the mobility of the water droplets is addressed. The numerical solution is based on solving Navier-Stokes equations for Newtonian liquids. The study includes the effect of interfacial forces with constant surface tension. The volume-of-fluid method is used to keep track of the deformation of free surfaces. A comprehensive set of simulations is conducted covering a wide range of density ratios, viscosity ratios, Capillary numbers, Ca, and Reynolds numbers, Re. Deformation of water droplets and their motion is characterized based on the maximum distance between the droplet surface and the channel wall. This characteristic length has been used to compare systems with different droplet and surrounding fluid properties. Among the parameters affecting the mobility of water droplets in a PEM fuel cell, the surface tension is found to have the most important effect. Copyright (c) 2005 John Wiley & Sons, Ltd.