Attenuated total reflection infrared (ATR-IR) spectroscopy in a flow-through cell was used to study the photocatalytic mineralization of malonic acid and succinic acid over P25 TiO2 in situ. The experiments were performed in water at concentrations of 1.5 x 10(-4) mol/L and pH 3.5 at room temperature. Changes on the catalyst surface were observed within a few minutes. The first step in the mineralization of malonic acid is a photo-Kolbe reaction of adsorbed malonate. Part of the resulting C-2 species is convened into oxalate and finally into carbon dioxide, and part desorbs from the surface. The branching ratio for the two pathways is 50:50. The mineralization reaction was also observed in the absence of dissolved oxygen, but at a slower rate. In the presence of dissolved O-18(2), labeled oxygen was incorporated into the adsorbed oxalate. A dominant pathway in the mineralization of succinic acid involves the transformation to oxalate via inalonate. Thus, it is proposed that a favored pathway for dicarboxylic acid mineralization is a photo-Kolbe reaction, followed by oxidation of the carbon-centered radical to a carboxylate, which corresponds to the overall formal shortening of the alkyl chain by one CH2 unit. (c) 2007 Elsevier Inc. All rights reserved.