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 1.5×10−4 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 C2 species is converted 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 18O2, labeled oxygen was incorporated into the adsorbed oxalate. A dominant pathway in the mineralization of succinic acid involves the transformation to oxalate via malonate. 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.