A flux-matching condition has been applied to determine whether O-2 reduction is rate-limiting under photocatalytic conditions for the degradation of CHCl3 at rutile TiO2 single-crystal electrodes. In this approach, the potential dependence of the photooxidation current density is compared to the potential dependence of the current density for O-2 reduction. The potential at which the oxidation and reduction fluxes are equal determines the operating potential and the steady-state flux that will flow through the crystal under no applied bias. If this flux-matching condition occurs when the cathodic flux equals the flux of photogenerated carriers, then the predicted quantum yield should approach unity; otherwise, recombination should be significant in the TiO2. Our measurements indicate that significant recombination will occur for the oxidation of typical organic molecules in H2O over a range of pH values. The data also indicate that Pt catalysis of O-2 reduction should be beneficial for the oxidation of organic molecules, as would the use of alternate electron accepters such as Fe(CN)(6)(3-). The O-2 reduction data and rotating disk electrode data collected in this work allow a quantitative comparison to theoretical estimates of the electron transfer rate constant for O-2 reduction at TiO2. We also present an elucidation of the previously published theoretical treatments of TiO2 charge transfer rate constants in view of the new data collected herein.