Six transition metal dioxides, M0O2, WO2, ReO2, RuO2, OsO2, and IrO2, have been examined as electrodes in H2SO4 solution. The oxides MoO2, WO2, ReO2, and RuO2 have broad current-potential profiles, indicating the formation of a surface layer which can exist over a range of compositions. Steady-state measurements of O2 reduction showed catalytic activities which were low compared to common O2 catalysts such as Pt, but of the same order as other oxide catalysts. Activities were lowest for WO2 and ReO2, which form resistive surface layers of a higher oxide. The only material studied which was sufficiently stable to allow measurement of both O2 evolution and O2 reduction was RuO2. The current-potential profiles of OsO2 and IrO2 are characterized by distinct changes of oxidation state. These two oxides were not sufficiently stable to allow the measurement of O2 reduction. © 1977, The Electrochemical Society, Inc. All rights reserved.