Cyclic voltammetry and steady-state techniques have been used to investigate the electrochemical behavior of single crystals of four conducting oxides of molybdenum, MoO2, Mo4O11, Mo8O23, and Mo9O26, and one conducting oxide of tungsten, WO2, in 1N H2SO4, and the ability of these oxides to serve as O2-reduction catalysts in H2SO4 solution. The behavior of the four oxides of molybdenum was found to be similar due to the formation of a surface layer, the composition of which is more highly dependent on the past history of the electrode potential than on the composition of the bulk of the crystal. Oxygen reduction on all of these oxides occurs, for a given current density, at over-potentials which are high compared to the more common O2-reduction catalysts such as Pt. The activity of WO2 toward O2 reduction is lower than that of MoO2 because of the greater stability of the less conductive higher oxide on the WO2 surface. Crystal orientation was found to have a substantial effect on the rate of O2 reduction. The rate was found to be higher on surfaces which do not include short metal-metal distances. The measurement of the kinetics and mechanism of O2 reduction is prevented by reactions of the electrode surface which occur in the same potential range. © 1977, The Electrochemical Society, Inc. All rights reserved.