The photocatalytic degradation of benzoic acid in water over Degussa P-25 TiO2 suspensions was studied. UVA irradiation at 365 nm was supplied by a medium pressure mercury lamp providing 25 mW cm(-2) light intensity. Experiments were conducted at benzoic acid initial concentrations between 25 and 150 mg L-1, catalyst loadings between 0.2 and 1 g L-1 and initial solution pH values between 2 and 10.6. Conversion increased with increasing catalyst loading up to about 0.6 g L-1 and it was favored at alkaline or neutral conditions but impeded at extremely acidic conditions. Increasing initial substrate concentration led to decreased benzoic acid conversion, which was found to follow a Langmuir-Hinshelwood kinetic expression. High performance liquid chromatography (HPLC) was employed to follow benzoic acid concentration profiles as well as to identify reaction by-products, while chemical oxygen demand (COD) and total organic carbon (TOC) analyses were carried out to assess the extent of mineralization. Benzoic acid hydroxylation by-products, namely 2-, 3- and 4-hydroxybenzoic acids as well as phenol were identified as reaction intermediates, although these contributed only a small fraction of the residual organic content. Although benzoic acid at 50 mg L-1 was not ecotoxic to marine photobacteria Vibrio fischeri, its photodegraded solution exhibited substantial toxicity, which, however, proved not to be due to the identified intermediates. (c) 2007 Elsevier B.V. All rights reserved.