Oxygen reduction at a gas-fed, porous CrO2 electrode bonded to a Dow Development Fuel Cell Membrane was measured at various values of pH. Bonding of the CrO2 to the membrane stabilizes the oxide. The activity for the reaction is strongest at pH = 14 and decreases monotonically with pH until it is completely suppressed in the range pH = 0 to pH = 3.94; it reappears for pH < 0. The pH of zero charge (isoelectric point) for CrO2 is 2.59. The data are interpreted in terms of an initial outer-sphere reaction followed by a surface exchange reaction that is rate-limiting. A change of the active site from Cr3+OH- at high pH to Cr4+OH- at pH < 0 is proposed; protonation of Cr3(+)OH(-) to Cr3+OH2 suppresses the reaction on this site at pH < 3.94.