The photocatalytic degradation of malic acid was studied under ambient conditions in an aqueous TiO2 suspension. The results demonstrated the presence of four distinct mineralization rate variations over the course of its degradation. Differences in the mineralization rate were governed by the dominant intermediate present in the solution at that time. Initial mineralization was rapid (similar to 175 mu g C/min) resulting from the swift extraction of a carbon from strongly adsorbed malic acid via the photo-Kolbe mechanism. The mineralization rate then slowed down (similar to 50-70 mu g C/min) with the continued formation of an intermediate product (believed to be malonaldehydic acid), malonic acid and acetic acid in the solution. The findings provide greater insight into the photocatalytic degradation mechanism, illustrating the importance of intermediates and their attributes in relation to the overall rate of organic removal. (C) 2010 Elsevier B.V. All rights reserved.