The rate-determining step of ethanol photocatalytic oxidation was identified to be the adsorption of O2 by an infrared (IR) spectroscopy coupled with mass spectrometry method. Dosing O2 during reaction showed that adsorption of O2 controls the accumulation of photogenerated electrons and the formation of acetate (CH3COO−ad), acyl species (CH3COad), acetaldehyde (CH3CHOad), CO2, and H2O. Accumulation of CH3COO−ad on the TiO2 surface slowed down the conversion of ethanol to CO2 and H2O. Removal of CH3COO−ad 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.