The effect of CH3CN at different concentrations on the oxygen reduction reaction (ORR) was investigated using a rotating ring-disk electrode (RRDE). CH3CN is found to have a high affinity to Pt/C and therefore leads to the suppression of the electrochemical surface area (ECSA) and the mass activity for the ORR. More H2O2 is collected on the ring during ORR, which indicates the shift from a 4-electron pathway to 2-electron pathway. Using the RRDE measurements and the Levich plots, the total charge transfer number is confirmed to decrease from 4 to approximately 3 at 0.2 V. The inhibition of the dual adsorption of the O-2 molecule due to the steric hindrance induced by the presence of CH3CN surface species is proposed as the cause of the shift in the reaction pathway. Oxidation/reduction of CH3CN is observed, in which both reversible oxidation/reduction (reactive adsorption products) and irreversible oxidation/reduction occur during the potential cycling process. For the reversible reaction, the products adsorb on the Pt surface and influence both the kinetics and reaction pathway. The hydrogenation of these products at lower potentials leads to higher coverage on the Pt, which leads to the generation of more H2O2 due to the spatial limitation. For the irreversible reaction, at least one of the products possess higher affinity to Pt than the adsorbed reversible reaction products, which leads to a continuous decrease in the ECSA during potential cycling and therefore to even lower kinetic performance for the ORR. Using a Tafel plot, it is found that none of the above-mentioned products change the rate determining step (RDS) of the ORR, in which the first charge transfer remains the rate determining step. (C) 2014 Elsevier Ltd. All rights reserved.