The photocatalytic degradation of four chlorinated aromatic compounds, namely pentachlorophenol (PCP), 2,4,-dichlorophenol (2,4,-DCP), 3,5,-dichlorophenol (3,5,-DCP), and 2,3,5,-trichlorophenol (2,3,5,-TCP), was investigated in the presence of an aqueous suspension of TiO2 irradiated with near-UV light. The toxicity of the transient species was compared to that observed for the parent compound. The comparison was carried out by monitoring the inhibition in bacterial respiration (Escherichia coil) by measuring the CO2 in the aqueous culture medium using flow injection analysis (FIA). Only in the case of PCP and 2,3,5,-TCP were the intermediates. identified by HPLC and GC/MS, found to be more toxic than the parent compound. The principal intermediates observed during the photocatalytic oxidation process of PCP were 2,3,5,6,-tetrachloro-1,4,-hydroquinone, 2,3,5,6,-tetrachloro-1,4,-benzoquinone and 2, 3,5,6,-tetrachlorophenol. In the case of 2,3,5,-TCP and 3,5,-DCP, the identified intermediates were 2,3,5,-trichloro-1,4,-hydroquinone and 2,5,dichloro-1,4,-hydroquinone, respectively. The intermediates of 2,4,-DCP were not identified. (C) 1997 Elsevier Science Ltd.31717281732