A transition metal cluster electrocatalyst based on Os x (CO) n was synthesized by pyrolysis of Os 3 (CO) 1 2 in 1,2-Dichlorobenzene (b.p.~180 o C) under inert atmosphere (N 2 ). The electrocatalytic parameters of the oxygen reduction reaction (ORR) for an Os x (CO) n catalyst were studied with a rotating disk electrode in 0.5MH 2 SO 4 electrolyte. The diffusion coefficient and solubility of O 2 in 0.5MH 2 SO 4 were calculated. Koutecky-Levich analysis of the linear voltamperometry data showed that the reaction follows first-order kinetics and the value of the Koutecky-Levich slope indicates a multielectron charge transfer during the ORR. The value of the Tafel slope obtained from the mass transfer corrected Tafel plots is 131mV/decade. The performance of the catalyst in a H 2 /O 2 PEM fuel cell cathode was evaluated and found to be nearly as good as that of Pt.