A pilot plant photocatalytic reactor of rectangular cross-section with a vertical stack of horizontally disposed UV lamps and an upwards flow of polluted water has been tested. Degussa VP Acroperl and Photox (Bradford) catalysts, both of large particle size (1 - 100 μ m), have been used so that catalyst/water separation with a hydrocyclone is feasible. A new photocatalytic rate equation has been used satisfactorily to assess results; it helps to explain why there is an optimum catalyst concentration in treating a solution containing 2-chlorophenol but not with one containing an orange dye, RO16.