The O-2 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]NO2OH systems, where N, the number of atoms in the Pt cluster, equals 18 and 25. Calculations indicate that dissociative O-2 electroadsorption on Pt(111) and molecular O-2 on Pt(100) are favored. As a result of the interactions of O-2 and OH in adjacent positions, hydroperoxide intermediates are formed on Pt(100) leading to the possibility of having H2O2 as product from the OERR.