The otherwise slow reaction between O-2 and decamethylferrocene (DMFc) or (SB)VIII+ (SB = Schiff base ligand) is significantly accelerated if all three reactants are present. In the absence of acid the reduction of O-2 proceeds stoichiometrically to yield the two oxo groups in (SB)(VO)-O-IV. In the presence of acid the reaction becomes catalytic and the O-2 is reduced to H2O. In the presence of excess O-2 the catalytic reaction ceases only when the supply of DMFc or acid is depleted. Investigation of the kinetics of the O-2 reduction processes led to a reaction scheme that accounts for much but not all of the kinetic data. (C) 2002 Elsevier Science B.V. All rights reserved.