The photocatalytic oxidation of acetone was studied on TiO2 powder and on a TiO2 monolayer catalyst anchored on porous Vycor glass (TiO2/PVG) using in situ solid-state nuclear magnetic resonance spectroscopy. Mesityl oxide was formed by aldol condensation after acetone was adsorbed on the TiO2 powder and its formation depended on the surface coverage of acetone. In comparison, acetone adsorption on the TiO2/PVG surface produced a chemisorbed propylene oxide surface species. Propylene oxide was strongly adsorbed on the TiO2/PVG surface and resistant to evacuation, while mesityl oxide was relatively weakly adsorbed on the surface and could be pumped away. No propylene oxide species was observed on the TiO2 powder. The photooxidation of acetone proceeded through two routes: The first was through the fragmentation of mesityl oxide to formic acid, which formed titanium formate and a surface OH group. Formate was further oxidized to the final oxidation product CO2. This process proceeded at a very slow rate. As a result, the oxidation rate of acetone was very slow on the TiO2 powder catalyst at high (greater than one monolayer) coverage. The second route was through the oxidation of the chemisorbed propylene oxide species, which was oxidized to acetic acid and then to CO2 at a much faster rate. The faster conversion rate of acetone observed on TiO2/PVG was due to the availability of this oxidation route.