The first results of a study of the oxygen reduction reaction on a rotating ring-disk electrode using single-crystal Pt disk electrodes are presented. The order of activity of Pt(hkl) in H2SO4 increased in the sequence (111) < (100) < (110). These differences are attributed to the structure sensitivity of (bi)sulfate anion adsorption and its inhibiting effect. Very small fluxes (I-R/N < 0.01I(D)) of peoxide were detected at the ring at potentials above 0 V (SCE) on all surfaces. However, below 0 V adsorbed hydrogen affected the partial reduction of oxygen to H2O(2) in the order (1I1) < (100) > (110), with 100% H2O2 formation on Pt(111).