A novel method for the ad hoc and real-time determination of the water balance in a proton exchange membrane fuel cell is presented. The method requires the anode side of the fuel cell to be operated in open-ended mode and to use dry, pure hydrogen as is typical for vehicular applications. In that case there is a linear relationship between the anode outlet velocity and the effective drag coefficient of water through the membrane, r(d), provided the stoichiometric flow ratio is sufficiently low (below xi = 1.2). The anode outlet velocity can then be directly measured e.g. by using hot wire anemometry, and this method provides a voltage signal that can be fed to the board computer of a fuel cell vehicle for PEMFC diagnosis. It is also shown that the nitrogen cross-over from cathode to anode has only a small effect on the anode outlet velocity. In addition to detecting the velocity, the relative humidity may be measured which is shown to be independent of the current density, but measurement techniques suffer from lower accuracy. It is argued that this method can also be applied to quantify fuel cell degradation. Finally, it is fundamentally shown that when operating the fuel cell in steady state mode at a hydrogen stoichiometric flow ratio as low as 1.03, the molar fraction of hydrogen in the gas mixture at the anode outlet is at least 50% which means that at steady state there will be no hydrogen starvation at the anode outlet. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.