The reversible deactivation rate during gas-solid heterogeneous photocatalytic oxidation of airborne dilute o-xylene over near-ultraviolet irradiated titanium dioxide catalyst depends on o-xylene concentration and the relative humidity of the reactant stream. Deactivation kinetics can be described by an exponentially decaying deactivation factor with time on stream. The exponential deactivation factor ct decreases linearly with decreasing o-xylene concentration and increasing relative humidity. The catalyst can be completely regenerated by passing humid air through the bed under continuous UV-irradiation, showing that the deactivation process is reversible. Infrared spectroscopy during UV irradiation and continuous flow of o-xylene-contaminated air indicates adsorption of o-xylene, along with formation of o-tolualdehyde, o-toluic acid, and benzoate ion, Infrared bands of the complete oxidation product carbon dioxide are also observed. The titania surface coverages of the reactant and aromatic partial oxidation intermediates are significantly higher for conditions of low relative humidity than for conditions of high humidity. Experiments performed at low relative humidity favor the buildup of o-xylene and o-toluic acid on the catalyst surface, and these species may be responsible for the apparent loss in activity. Regeneration of the catalyst deactivated during the photocatalytic oxidation process run at high humidity requires much shorter catalyst treatment time than that for the catalyst deactivated by photocatalytic oxidation run at low humidity. This phenomenological behavior suggests that hydroxyl radicals play a significant role in both the oxidation and the regeneration Processes,