Titanium dioxide thin film photo-semiconductor electrodes prepared by chemical vapour deposition (CVD) are compared for the photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of 4-chlorophenol (4-CP) in aqueous solution. The efficiencies of PC degradation of 4-CP are low, but are found to be improved significantly by PEC with an applied potential. The effects of the film properties, particularly film thickness and crystalline form, on degradation rate by PEC have also been studied. For films deposited at 370 and 400 degreesC it is suggested that differences in behaviour could be related to depletion layer widths.