The kinetics of the oxygen reduction reaction (ORR) on Pt(hkl) surfaces is found to vary with crystal face in a different manner depending on the electrolyte. In perchloric acid, the variation in activity at 0.8 to 0.9 V is relatively small between the three low index faces, with activity increasing in the order (100) < (110) approximate to (111). A similar structure sensitivity was observed in KOH, increasing in the order (100) < (110) < (111), but with larger differences. In sulfuric acid, the variations in activity with crystal fate were much larger, with the difference between the most active and the least active being about two orders of magnitude, and increased in the opposite order (111) << (100) < (110). The variations in activity with crystal face in perchloric acid and KOH arise from the structure sensitive inhibiting effect of OHads i.e., a strongly inhibiting effect on (100) and smaller effects on (110) and (111). The variations in activity with crystal face in sulfuric acid arise from highly structure specific adsorption of sulfate/bisulfate anions in this electrolyte, which has a strongly inhibiting effect on the (111) surface. The crystallite size effect for the ORR reported for supported Pt catalysts in sulfuric acid at ambient temperature is fully explained by applying our single crystal results to classical models of the variation in particle shape with size.