Alkyl derivatives of uracil, namely, 5,6-dimethyluracil, 5-ethyluracil, and 5-methyluracil accelerate the rate of electrochemical reduction of oxygen on Pt catalysts in aqueous 0.5 M sulfuric acid. Two other uracil derivatives, namely, 1,3-dimethyluracil and 2,4-dimethoxypyrimidine, and uracil itself, also increase the rate of the oxygen reduction reaction, but to a much lesser degree. The organic molecules were adsorbed on the electrode surface from dilute solutions of 0.5 mM in the electrolytes. The organic adsorbates shift the open-circuit potential of the Pt electrode to more positive values and generate higher oxygen reduction currents at lower overpotentials. Replacement of the adsorbed anions from the surface by the organic molecules facilitates oxygen reduction. Partial electron transfer between the electrode and some of the adsorbed molecules also plays a role in their catalytic behavior. The degree of enhancement by the organic adsorbates reported here for the rough Pt electrode is somewhat different from that found on a smooth, polycrystalline electrode. These differences are explained based on the effect of surface roughness and the time-dependent variation of the diffusion layer thickness caused by the diffusion of oxygen to the electrode surface.