The first results of a study of the oxygen reduction reaction on a rotating ring-disk electrode using single-crystal Pt disk electrodes in alkaline solution are presented. The order of activity of Pt(hkl) in 0.1M KOH increased in the sequence (100) < (110) < (111) for both oxygen and peroxide reduction. These differences are attributed to the structure sensitivity of hydroxyl anion (OH.) adsorption on Pt(hkl) and its inhibiting (site blocking) effect on oxygen kinetics. Two different types of OHads occur on Pt(100) and Pt(110), a reversible OHIV, and an irreversible OHir, while only the reversible form is formed on Pt(111). Very small fluxes (I-R/N << 0.011(D)) of peroxide were detected at the ring at potentials where the surfaces have only the reversible form of OHads. More peroxide (I-R/N < 0.03I(D)) was found with the Pt(100) and Pt(110)surfaces in the potential region where irreversible adsorption occurs. The mechanism of O-2 reduction was also affected by adsorbed hydrogen with increased formation of peroxide ions in this potential region; correspondingly, adsorbed hydrogen was found to have an inhibiting effect on peroxide reduction, the effect decreasing in the order (111) > (100) >> (110).