An experimental procedure was developed to measure oxygen reduction reaction kinetics in subfreezing polymer electrolyte membrane (PEM) fuel cells. The procedure was also used to measure kinetics at temperatures above 0 degrees C, and results compared to those collected with a traditional kinetic measurement technique. In general, because of brief time durations in which PEMFCs can be operated below freezing temperatures, short equilibration times were required and thus, enhanced catalyst activity was observed. At progressively lower subfreezing temperatures, suspected mass transport or uncompensated ohmic losses resulted in nonlinear Tafel plots, which at lower decades of current density become linear with a slope close to that predicted by Tafel kinetics, 2.303RT/alpha F-c. Consistent with results of other researchers at nonfrozen conditions, low water (or ice) content in the fuel cell results in lower catalyst activity and performance at subfreezing temperatures. Cyclic voltammograms indicate that the rate of oxide formation decreases at subfreezing temperatures and low water contents, indicating proton activity as a likely reason for reduced catalytic activity. Arrhenius plots of current density at a constant overpotential are linear (constant activation energy) over the temperature range from 55 to-40 degrees C, indicating no fundamental change in reaction mechanism at subfreezing temperatures. (c) 2007 The Electrochemical Society.