Although acetone is commonly used to evaluate the performance of oxide photocatalysts, little is known about the mechanistic details of its photo-oxidation. This study provides insights into the photodecomposition of adsorbed acetone using the (110) face of rutile TiO@sub 2@ as a model photocatalyst. In the absence of UV light, acetone desorbs from the clean TiO@sub 2@(110) surface without decomposition, exhibiting strong coverage-dependence in its temperature programmed desorption (TPD) peak that shifts from 350 K to below 250 K as the monolayer is populated. Acetone molecules desorbing at 350 K constitute about 0.25 ML and exhibit H/D exchange with surface hydroxyl groups. On the other hand, coadsorbed water displaces about 0.75 ML of the acetone monolayer into physisorbed states, but does not influence the remaining 0.25 ML that constitutes the 350 K TPD peak. These strongly bound acetone molecules are not associated with oxygen vacancies. Virtually no photodecomposition is observed in the absence of gas phase O@sub 2@. Exposure to UV light in gas phase O@sub 2@ results in conversion of acetone to acetate via cleavage of a carbonyl-methyl bond. A similar reaction mechanism occurs in the dark during the coadsorption of acetone and molecular oxygen preadsorbed at oxygen vacancies, suggesting that acetone photodecomposition is facilitated through the excited electron channel (e.g., via reaction with an O@sub 2@@super -@ species) and not through oxidation by valence band holes. Photodesorption measurements reveal that the methyl group is ejected from the surface at 200 K but is retained on the surface at 100 K presumably by conversion into formate based on the absence of likely C@sub 1@ or C@sub 2@ species in TPD. The acetone photodecomposition cross section increases with increasing acetone coverage, but decreases with coadsorbed water. @FootnoteText@ This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.