The theoretical contributions to overall cell voltage in a proton exchange membrane fuel cell (PEMFC) were calculated, accounted for, and compared to measured IR free cell voltage as relative humidity (RH) was varied from 30-110%. The intrinsic oxygen reduction reaction (ORR) kinetics in a PEMFC, with regards to water and proton activity, were shown to be independent of RH above 50-60%, but significant losses in ORR kinetics were observed at lower RH values. The losses amounted to ca. 20 mV at 30% RH, corresponding to an approximate two-fold decrease in the ORR exchange current density. The dependence of ORR kinetics on water and proton activity is consistent with reduced H+ activity at low RH. © 2005 The Electrochemical Society. All rights reserved.