Stopped-flow kinetic measurements, UV-vis spectroscopy, rotating disk voltammetry, and quantum chemical calculations are used to clarify the role of water in the homogeneous two-electron reduction of O-2 to H2O2 in 1,2-dichloroethane (DCE) using ferrocene (Fc) as an electron donor, tetrakis(pentafluorophenyl)boric acid (HTB) as a proton donor, and [5,10,15,20-tetraphenyl-21H,23H-porphine]cobalt(II) ((CoTTP)-T-II) as a catalyst. Kinetic analysis suggests that the reaction is controlled by the intramolecular proton coupled electron transfer to the O-2 molecule coordinated to the metal center producing the O2H center dot radical. This rate-determining step is common to both the O-2 reduction by Fc catalyzed by (CoTPP)-T-II and the O-2 reduction by (CoTPP)-T-II itself. Experimental data point to the competitive coordination of water to the metal center leading to a strong inhibition of the catalytic reaction. In agreement with this finding, quantum chemical calculations indicate that water is bound to the metal center much more strongly than triplet O-2. A similar effect is demonstrated also for the O-2 reduction catalyzed by the porphyrin free base (H2TPP), though its rate is lower by 2 orders of magnitude.