Through the use of a water balance experiment, the electro-osmotic drag coefficients of Nafion 115 were obtained under several conditions (as a function of water content and thermodynamics conditions). For the cases when the anode was fully hydrated (corresponding to water content lambda approximate to 14 in the adjacent membrane) and the cathode suffered from drying when dry air was supplied (A approximate to 2), the electro-osmotic drag coefficients varied from 0.82 (+/- 0.06) to 0.50 (+/- 0.03) H(2)O/H(+) when the current density varied from 0.4 to 1.0 A cm(-2) (95% confidence level). When the current density increased, the electro-osmotic drag coefficient decreased. When the water content at the anode increased from A approximate to 5 to lambda approximate to 14, the cathode was supplied with dry air (lambda approximate to 2), and the fuel cell discharged constant current density at 0.6 A cm(-2), the electro-osmotic drag coefficient increased from 0.44 (+/- 0.06) to 0.68 (+/- 0.06) H(2)O/H(+) (95% confidence level). Higher relative humidity gas leads to a higher electro-osmotic drag coefficient at constant current density. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.