A series of modified carbon paste electrodes were constructed by incorporation of 1,4-naphthoquinone and some of its derivatives in a graphite powder-mineral oil matrix. It has been shown by cyclic voltammetry, rotating disk electrode voltammetry and chronoamperometry that these electrodes can catalyze the reduction of O-2 to H2O2 in aqueous medium (pH >2). It has been found that in the optimum condition, the reduction of dioxygen at the surface of such electrodes occurs at potentials about 350 to 550 mV less negative than at an unmodified carbon paste electrode. The heterogeneous rate constants for the reduction of O-2 at the surface of these modified carbon paste electrodes were determined by use of Koutecky-Levich plots. In addition, the apparent diffusion coefficients of modifiers in mineral oil, O-2 in solution and the effective area of carbon paste electrodes have also been estimated.