Transient responses of conventional carbon-supported Pt nanoparticle (Pt/C) and 3M's nano structured thin film (NSTF) oxygen reduction electrodes are reported. The ultrathin 0.3 mu m thick NSTF electrode exhibits less-than-zero voltage, i.e. a failed up-transient, when current is increased instantaneously from 0.02 to 1.0 A/cm(2) at 80 degrees C and the channel RH is greater than 80%. In contrast, under the same conditions, a 10 mu m thick Pt/C electrode exhibits robust transient response. A transient model, coupled with experiments, is deployed to delineate the water transport dynamics and ensuing voltage response. It is found that thin electrodes, due to their low water storage capacities, flood with liquid water rendering negative cell voltages during transients. Effects of operating conditions and material properties such as membrane and electrode thickness on the transient response are investigated. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3574368] All rights reserved.