Acetic acid decomposes photocatalytically on TiO2 at room temperature in an inert atmosphere through two parallel pathways. In one pathway, acetic acid decomposes to gas-phase CO2 and apparently forms hydrogen and methyl groups, which combine on the surface to form CH4. In the other pathway, acetic acid extracts oxygen from the TiO2 lattice to form adsorbed H2O and gas-phase CO2 and C2H6. The extracted oxygen is replenished by diffusion from the bulk in an inert atmosphere or by gas-phase O-2. The formation of CH4 and CO2 in the first pathway does not consume lattice oxygen. The first step in photocatalytic decomposition (PGD) of acetic acid appears to be dissociation of the O-H bond, producing surface acetates. However, molecularly adsorbed acetic acid reacts at the same rate and with the same selectivity as surface acetates. Only the alpha-carbon forms CO2 during PGD. When gas-phase O-2 is present, adsorbed methyl groups oxidize before they are hydrogenated to CH4. The oxidizing agent during photocatalytic oxidation (PCO) is different from that during PCD and is most likely adsorbed oxygen. Adsorbed oxygen reacts with acetic acid in a different pathway from the two reactions observed for PCD, so a Mars Van Krevlen mechanism for PCO appears unlikely. The TiO2 surface is not homogeneous and some surface sites are more active during both PGD and PCO. Go-adsorbed water increases the rate of CH4 formation, apparently by reacting with CH3(ads) to form CH4, but in contrast to adsorbed O-2, water does not react with acetic acid in a separate pathway that is different from those observed for PCD without water.