This paper presents a scheme for estimating membrane water content in polymer electrolyte membrane (PEM) fuel cells from voltage, current, temperature, and several pressure measurements. The approach is to exploit the resistive voltage drop which is closely associated with membrane water content. To distinguish this resistive drop from other voltage losses we make use of a well-developed fuel cell voltage model that characterizes each loss term, as well as the open circuit voltage. The unmeasured hydrogen and oxygen partial pressure values in the open-circuit model are estimated with a variant of the hydrogen and oxygen observers developed in . Preliminary experimental results, obtained at the Connecticut Global Fuel Cell Center, are presented and discussed.