Water balance in a polymer electrolyte membrane fuel cell (PEMFC) was investigated by measurements of the net drag coefficient under various conditions. The effects of water balance in the PEMFC on the cell performance were also investigated at different operating conditions. Experimental results reveal that the net drag coefficient of water through the membrane depended on current density and humidification of feed gases. It was found that the net drag coefficient (net number of water molecules transported per proton) ranged from -0.02 to 0.93, and was dependent on the operating conditions, the current load and the level of humidification. It was also found that the humidity of both anode and cathode inlet gases had a significant effect on the fuel cell performance. The resistance of the working fuel cell showed that the membrane resistance increased as the feed gas relative humidity (RH) decreased. The diffusion of water across Nation membranes was also investigated by experimental water flux measurements. The electro-osmotic drag coefficient was evaluated from the experimental results of water balance and diffusion water flux measurements. The value of electro-osmotic drag coefficient, ranging from 1.5 to 2.6 under various operating conditions, was in agreement with literature values. The electro-osmotic drag coefficient, the net flux of water through the membrane and the effective drag as a function of operating conditions will also provide validation data for the fuel cell modeling and simulation efforts. (c) 2005 Elsevier B.V. All rights reserved.