The O2 electroreduction reaction (OERR) on Pt behaves as a structure sensitive reaction whose peroxo intermediates are formed in a greater extent on Pt(100) than on Pt(111) surfaces. A semiempirical quantum chemistry interpretation of this behavior is attempted on the basis of the study of [Pt]N02OH systems, where N, the number of atoms in the Pt cluster, equals 18 and 25. Calculations indicate that dissociative O2 electroadsorption on Pt(111) and molecular O2 on Pt(100) are favored. As a result of the interactions of O2 and OH in adjacent positions, hydroperoxide intermediates are formed on Pt(100) leading to the possibility of having H2O2 as product from the OERR.