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Photochemical degradation of 4-chlorophenol (4-CP) as a model wastewater contaminant with three methods: UVfriO2, UV/H20, and UVfFiO2/H202 (2 > 310nm) has been investigated and compared to the dark Fenton reaction and to direct photolysis. A UV-irradiated combination of TiO2 and H202 was found to be the most effective degradation method for TOC (total organic carbon). 4-CP was degraded most rapidly by the dark Fenton reaction. In the heterogeneous process on illuminated TiO2 without H 202, only small amounts of by-products were formed during irradiation in contrast to homogeneous processes where H,O2 was involved. During UV/H202 and Fenton experiments, coloured by-products appeared during irradiation and degradation rates of 4-CP and TOC showed strong differences between homogeneous and heterogeneous processes.