In this study, the effect of water balance variation on electro-osmotic flow in a fuel cell with elliptical cross section has been investigated experimentally. By changing the humidity on both sides of the fuel cell, required data for electro-osmotic flow and back diffusion were calculated. In addition, using the balance equations for water's mass at anode and cathode, the values of the electro-osmotic flow and back diffusion in different current densities were found. Results show that variations of the electro-osmotic flow and back diffusion change linearly with current density and slope of the curves increase by increasing in humidity. Besides, after a certain value, humidity raise has a negligible effect on curves of electro-osmotic flow and back diffusion. In other words, there is an optimum value for humidity at anode and cathode side. Based on the results, it was determined that increasing in humidity at anode side has a more desirable effect than the cathode. For instance, when cathode and anode had the humidity level of 0% and 70% respectively, maximum current density of the fuel cell was recorded about 0.86 A/cm(2) but when cathode and anode had the humidity level of 70% and 0% respectively, maximum current density of the fuel cell was obtained about 0.512 A/cm(2). Using available equations, electro-osmotic coefficients were also calculated and their variations with respect to current density and humidity level were outlined. Results show that electro-osmotic coefficient remains constant with respect to current density, while it increases with increasing the humidity. In this work, electro-osmotic coefficients varied between 0.636001 and 1.632476, which were in a good agreement with the values in other related papers. (C) 2016 Elsevier Ltd. All rights reserved.