The O-2-electroreduction reaction was studied at polycrystalline and [(111)-type and (100)type] facetted Pt rotating disc electrodes in O-2-saturated acid and alkaline solutions at 25 degrees C. From the Tafel plots, two linear regions, one at a low (Icd) and another at a high (hcd) current density range, with Tafel slopes, (b(T))(lcd) = - 0.06 V decade(-1) and (bT)(hcd) greater than or equal to - 120 V decade(-1) were found. In contrast to (bT)(lcd) the value of (b(T))(hcd) showed a dependence on the type of Pt electrode, the solution pH and the O-2 partial pressure. Otherwise, rotating ring-disc electrode (rrde) data on both acid and alkaline solutions showed that the amount of H2O2 produced on (100)-type facetted Pt was greater than that formed on the other Pt surfaces. The analysis of these data has provided new evidence that the O-2-electroreduction to H2O for the three Pt electrodes proceeds principally via the 4-electron pathway, but for (100)-type facetted Pt a significant contribution of the 2-electron pathway has also been found.