The electrochemical behaviour of benzofuroxan and o-benzoquinone dioxime on bare platinum and Pt surfaces covered by heavy metal monolayers deposited at underpotentials was studied in aqueous acid solutions. It was found that II, Pb and Bi adsorbates markedly catalyse the reduction of benzofuroxan. The enhancement of the overall reduction process has been interpreted in terms of the change of the reduction mechanism from chemical mechanism on bare platinum to direct electrochemical mechanism of F't surfaces covered by the underpotential monolayers. The reduction waves of benzofuroxan and o-benzoquinone dioxime on Pt/M,, surfaces were found to be kinetically controlled. The kinetic nature of the current was attributed to the dehydration reactions of tbe intermediate bydroxylamines formed in the course of the electrode reaction. It wasalso found that underpotential layers markedly improve the reversibility of the twoelectron oxidation of o-benzoquinone dioxime. This catalytic et&t has been interpreted in.,terms of the change of the reduction mechanism from an "inner sphere" mechanism Involving adsorbed Intermediates on pure platinum to an "outer sphere" one without complications from the adsorption of the reacting molecules bn Pi/&&, surfaces.