A transition metal cluster electrocatalyst based on Os(x)(CO)(n) was synthesized by pyrolysis of Os(3)(CO)(12) in 1,2-Dichlorobenzene (b.p. approximate to 180 degreesC) 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 0.5 M H(2)SO(4) electrolyte. The diffusion coefficient and solubility of O(2) in 0.5 M H(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 131 mV/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.