The oxygen reduction reaction (ORR) in acid and alkaline solutions is investigated on thin Pd films (0 < theta(Pd) < 1.5 ML) supported on a Pt(111) electrode. In 0.1 M KOH, the general form of the current vs Theta(Pd) relationship for the ORR has a volcano shape, with the maximum catalytic activity exhibited by a surface modified with 1 ML of Pd. However, if the amount of Pd deposited exceeds 1 ML, the reaction is inhibited and 1.5 ML of Pd are less active than unmodified Pt(111). In 0.1 M HClO4, none of the Pd films are more active than Pt(111). In solution containing Cl- (ca. 10(6)-10(-5) M), the inhibition on the Pt(111)-Pd electrode is much more severe than on bare Pt(111) due to stronger anion adsorption on Pd. We present an interpretation of the kinetics of the ORR on Pt modified by thin Pd films in alkaline and acid solution based on the electronic properties of the Pd films and how the electronic properties effect specific anion adsorption, the formation of OHad and the adsorption of the reaction intermediates.