The rate-determining step of ethanol photocatalytic oxidation was identified to be the adsorption of O-2 by an infrared (IR) spectroscopy coupled with mass spectrometry method. Dosing O-2 during reaction showed that adsorption of O-2 controls the accumulation of photogenerated electrons and the formation of acetate (CH3COOad-), acyl species (CH3COad), acetaldehyde (CH3CHOad), CO2, and H2O. Accumulation of CH3COOad- on the TiO2 surface slowed down the conversion of ethanol to CO2, and H2O. Removal of CH3COOad- from the TiO2 surface holds the key to accelerating the rate of ethanol photocatalytic oxidation. This Study bridges the gap between results of nanosecond and millisecond transient absorption studies and those of minute scale photocatalytic oxidation studies.