The effect of surfactants on the electroreduction of O-2 to H2O2 was investigated by cyclic voltammetry and batch electrolysis on vitreous carbon electrodes. The electrolytes were either 0.1 M Na2CO3 or 0.1 M H2SO4 at 295 K, under 0.1 MPa O-2. Electrode kinetics and mass transport parameters showed the influence of surfactants on the O-2 electroreduction mechanism. The cationic surfactant (Aliquat 336(R), tricaprylmethylammonium chloride), at mM levels, increased the standard rate constant of O-2 electroreduction to H2O2 15 times in Na2CO3 and 1900 times in H2SO4, to 1.8 x 10(-6) m s(-1) and 9.9 x 10(-10) m s(-1), respectively. This effect on the reaction rate might be due to an increase of the surface pH, induced by the Aliquat 336(R) surface film. The nonionic (Triton X-100) and anionic (sodium dodecyl sulfate) surfactants retarded the O-2 electroreduction, presumably by forming surface structures, which blocked the access of O-2 to the electrode. Ten hour batch electrosynthesis experiments performed at 300 A m(-2) superficial current density, 0.1 MPa O-2, 300 K, on reticulated vitreous carbon (30 ppi), showed that compared to the values obtained in the absence of surfactant, mM concentrations of Aliquat 336(R) increased the current efficiency for peroxide from 12% to 61% (0.31 M H2O2) in 0.1 M Na2CO3 and from 14% to 55% (0.26 M H2O2) in 0.1 M H2SO4, respectively.