The catalytic activity of Pt and Pt alloys for oxygen reduction reaction (ORR) is affected by the formation of Pt oxides at high potentials. The transient ORR and oxide behaviors of two PEM fuel cell catalysts (46 wt% Pt/V and 30 wt% PtCo/HSC) were studied by chronoamperometry at fixed potentials. Both rotating disk electrodes (RDEs) and membrane electrode assembly (MEAs) of fuel cells were used to compare how the transient formation of oxides and its effects on ORR activity may differ between these two common testing environments. Similar trends of recoverable activity degradation were observed for the RDE and MEA tests. Higher oxide coverages and faster oxide growth rates were observed for the alloyed PtCo/HSC catalyst in comparison to the Pt/V catalyst. The alloy showed higher initial activity but faster degradation rates over the brief time scales of these experiments. Results contradict the purely simple size effect as the explanation for the differences between the Pt and PtCo catalysts. We found that activity is not simply a function of oxide coverage, and is affected by additional mechanisms and conditions. (C) 2013 The Electrochemical Society.