The effects of TiO2 surface loading and light conditions on the photocatalyzed degradation of 4-chlorophenol (4-CP) were examined, in order to address deficiencies in previous studies in which these issues were not examined satisfactorily. It is proposed that these factors explain differences in the reported reaction intermediate concentrations. Increasing the loading of TiO2 in a slurry has little effect on the rate of 4-CP disappearance. However, the concentration of 4-chlorocatechol (4-CC), a primary degradation product, decreases with increasing TiO2 loading. Zero order kinetics in terms of 4-CP were observed for 4-CP degradation. The rate of mineralization increased with increasing TiO2 concentration. The rate of mineralization relative to 4-CP degradation was more rapid under conditions of lower light intensity. Significant adsorption of 4-CC on Degussa P25 TiO2 was measured and these data are fitted well by a Langmuir isotherm. Adsorption of 4-CC accounts for most of the changes in 4-CC concentration at different TiO2 loadings. An increased proportion of surface oxidation reactions at higher TiO2 loadings and lower light intensity may explain the higher rates of mineralization in thicker slurries. The quantum yield of reaction of 4-CP increased with decreasing wavelength of light from phi = 0.01, at lambda = 360 nm to phi = 0.07, at lambda = 300 nm. A relationship of d[4-CP]/dt proportional to I-0.8 was measured.