Cyclic voltammetry is used to study the role of 5,10,15,20-tetraphenyl-21H,23H-porphine (H2TPP) in the reduction of molecular oxygen by decamethylferrocene (DMFc) at the polarized water|1,2-dichloroethane (DCE) interface. It is shown that this rather slow reaction proceeds remarkably faster in the presence of tetraphenylporphyrin monoacid (H3TPP+) and diacid (H4TPP2+), which are formed in DCE by the successive transfer of two protons from the acidified aqueous phase. A mechanism is proposed, which includes the formation of adduct between H3TPP+ or H4TPP2+ and O2 that is followed by electron transfer from DMFc to the adduct leading to the observed production of DMFc+ and to the regeneration of H2TPP or H3TPP+, respectively.