Proton-exchange-membrane fuel cells (PEMFC) are highly dependent on operating conditions, such as humidity and temperature. This study employs electrochemical impedance spectroscopy (EIS) to measure the effects of operating parameters on internal proton and electron transport resistance mechanisms in the PEMFC. Current-density experiments have been performed to measure the power production in a 25cm(2) Nafion 117 PEMFC at varying operating conditions. These experiments have shown that low humidity and low temperature contribute to decreased power production. EIS is currently employed to provide a better understanding of the mechanisms involved in power production by calculating the specific resistances at various regions in the PEMFC. Experiments are performed at temperatures ranging from 30 to 50 degrees C, feed humidities from 20 to 98%, and air stoichiometric ratios from 1.33 to 2.67. In all experiments, the hydrogen feed stoichiometric ratio was approximately 4.0. EIS is used to identify which transport steps limit the power production of the PEMFC over these ranges of conditions. The experimental data are analyzed via comparison to equivalent circuit models (ECMs), a technique that uses an electrical circuit to represent the electrochemical and transport properties of the PEMFC. These studies will aid in designing fuel cells that are more tolerant to wide-ranging operating conditions. In addition, optimal operating conditions for PEMFC operation can be identified.