Oxygen reduction has been studied in 0.1 M HClO4 on the platinum low index surfaces, employing a hanging meniscus rotating-disk technique. A special cleaning procedure has been developed to remove impurities from the surface prior to the oxygen reduction measurements. Oxygen reduction was found to be sensitive to the crystallographic orientation of the platinum electrode surface. The activity for oxygen reduction discerned from the half-wave potential decreases in the sequence (110) > (111) > (100). The reaction proceeds on all well-ordered low index planes with exchange of four electrons per O-2 molecule. Tafel slopes of -120 mV per decade at high current densities and -60 mV/per decade at low current densities were found for all three planes at room temperature. Oxygen-containing species chemisorbed on platinum appear to be the cause of the change in the Tafel slope.