As part of the study on TiO2-heterogeneous photocatalytic degradation of citric acid (Cit), degradation of 3-oxoglutaric acid (OGA, 5 mM, pH 2.8), one of the main intermediates of Cit, was analyzed. As reactive beta-ketodicarboxylic acid, OCR decays rapidly in ambient dark conditions to acetoacetic acid and acetone. The photocatalytic reaction over TiO2 under UV light is thus affected by this thermal pathway and, in addition to the products found in dark conditions, other carboxylic acids like lactic, pyruvic, glyoxylic and acetic acid were detected and quantified. TOC decrease is proven due mostly by the thermal reaction. Kinetic parameters for both thermal and photochemical reactions were obtained from the experimental data, and mechanisms for the TiO2-photocatalytic degradation were proposed for OCR and its intermediates. The interplay between OCR thermal and photocatalytic decay is verified, a subject not very much described in the literature. (C) 2010 Elsevier B.V. All rights reserved.