Dioxygen accelerates back electron transfer (BET) processes between a fullerene radical anion (C-60) and a radical cation of zinc porphyrin (ZnP) in photolytically generated ZnP.+-C60(.-) and ZnP.+-H2P-C60(.-) radical ion pairs. The rate constant of BET increases linearly with increasing oxygen concentration without, however, forming reactive oxygen species, such as singlet oxygen or superoxide anion. When ferrocene (Fc) is used as a terminal electron donor moiety instead of ZnP (i.e., Fc-ZnP-C-60), no catalytic effects of dioxygen were, however, observed for the BET in Fc(+)-ZnP-C60(.-), that is, from C60(.-) to the ferricenium ion. In the case of ZnP-containing C-60 systems, the partial coordination of O-2 to ZnP.+ facilitates an intermolecular electron transfer (ET) from C60(.-) to O-2 This rate-determining ET step is followed by a rapid intramolecular ET from O-2(.-) to ZnP.+ in the corresponding O2(.-)ZnP(.+) complex and hereby regenerating O-2. In summary, O-2 acts as a novel catalyst in accelerating the BET of the C60(.-)-ZnP.+ radical ion pairs.