The photocatalytic generation of H₂ and hydrocarbons from aqueous acetic acid has been studied employing TiO₂ P25-based photocatalyst particles. The effect of pH on the distribution of reaction products and their formation rates during the photochemical as well as the photoelectrochemical oxidation of acetic acid has been investigated. The photocatalytic formation rates for H₂ and hydrocarbons were found to be higher for Pt-loaded TiO₂ than for bare TiO₂. The major reaction products resulting from the photocatalytic decomposition of aqueous acetic acid, as determined quantitatively in the gas phase, were found to be H₂, CO₂, and CH₄. Furthermore, traces of C₂H₆, C₃H₈, CO, CH₃CHO, HCHO, CH₃OH, C₂H₅OH, and HCOOH were also detected. After 15 h of illumination, the average formation rates of H₂, CO₂, CH₄, and C₂H₆ evolved at pH 2 were found to be 22, 65, 35, and 2 µmol/h, respectively. The ratio of H₂ to hydrocarbons strongly depends on the pH values, i.e., at acidic pH the ratio of H₂ to CH₄ formation was found to be 0.6. On the contrary, at neutral and basic pH values negligible amounts of hydrocarbons were formed and H₂ was found to be the main product with formation rates of 12 and 5 µmol/h at pH 7 and 11, respectively. It is therefore assumed that the hydroxide ion has a significant effect on the reaction pathways. Due to the fact that methanol and ethanol are formed as reaction products, water or hydroxide ions are apparently required for the formation of the major oxidizing agent, that is the hydroxyl radical. Herein, a detailed mechanism for the photocatalytic decomposition of acetic acid at different pH values is presented.