Ultraviolet light dramatically increases the rate of isotope exchange between gas-phase O-2 and water adsorbed on TiO2 at room temperature, but it does not affect the rate of CO2-water exchange. Both ethanol and acetaldehyde, when coadsorbed with (H2O)-O-18, dramatically decrease the rate of O-2 exchange, but not CO2 exchange, with adsorbed (H2O)-O-18. This decrease is attributed to a combination of competition for adsorbed oxygen between exchange and photocatalytic oxidation of the adsorbed organic and blocking of the oxygen adsorption sites by the organic. The same oxygen species participate in O-2-(H2O)-O-18 exchange and photocatalytic oxidation.