Transforming pollution into valuable compounds and more especially into energy is an important challenge for a sustainable future. The objective of our work was to better understand the mechanism of the photocatalytic degradation of acetic acid (AA) in the gas phase under an air or oxygen-free atmosphere, by comparing the disappearance, mineralization and selectivity of the intermediate products formed in the presence of non-doped TiO2. It was found that in both environments decarboxylation is the first step of AA degradation. However, the fate of the methyl group depends on the carrier gas and gas phase concentration. In air, at low AA molar flow the methyl group is oxidized into methanol, formaldehyde, and CO2 with no formic acid detected. At molar flows larger than 0.55 p,moli min, corresponding to the saturation of the TiO2 surface, ethane is formed due tothe combination of two methyl groups. In an O-2-free atmosphere, three new products were detected: methane (CH4), acetone (CH3COCH3) and acetaldehyde (CH3COH). However, the use of (CH3COOH)-C-13 shows that acetone and acetaldehyde do not arise from the reduction of the carboxylic group. Under air, a mechanism explaining the formation of methanol, formaldehyde and ethane occurring on the surface of TiO2, involving either a direct oxidation of AA with the holes or an oxidation with the hydroxyl radicals was proposed, together with the regeneration of the TiO2 surface and the formation of H2O2. (C) 2017 Elsevier B.V. All rights reserved.