The effect of C2H2 on the oxygen reduction reaction (ORR) for a commercial Pt/C catalyst was investigated using rotating ring disk electrode (RRDE) technique in acidic solution. This study was undertaken to provide insight into the mechanism of C2H2 contamination of the cathodes in proton exchange membrane fuel cells. The cyclic voltammetry results show a high C2H2 coverage on the Pt surface and an almost complete loss of the electrochemical surface area in the presence of 0.14mM C2H2. The RRDE was used to measure the ORR polarization curves and H2O2 production in air. The introduction of C2H2 shifts the ORR onset potential in the negative direction by 330 mV, and no limiting current can be observed in the potential scan window. The significant retardation of the ORR is associated with the complete loss of the ECSA, as the adsorption of C2H2 on the Pt sites results in the inhibition of both Hupp and O-2 adsorption. Furthermore, it is proposed that C2H2 adsorption also has an impact on the adsorption configuration of O-2 molecules; the Pauling configuration prevails due to the spatial limitations imposed by the presence of adsorbed C2H2 on Pt. As a result, both the ring-disk and Koutecky-Levich measurements show a shift in the reaction pathway from a 4- to a 2-electron reduction: the H2O2 production increases and the charge transfer number decreases. The ORR rate determining step is observed to be shifted from the first electron transfer to other possible steps. This change is confirmed by the Tafel slope measurement, which increases significantly and is most likely due to the changes in the adsorption energy of O-2. Nearly complete recovery of the performance is attainable by stopping the C2H2 exposure. The unrecovered performance is attributed to the remaining surface adsorbates. (C) 2014 Elsevier Ltd. All rights reserved.