Photocatalytic degradation of one of the most widely used cotton dyes, namely reactive red 22 (RR 22), was investigated in the presence of a thin film of ZnO photocatalyst using a thin film flat bed flow photoreactor under solar radiation. The effects of reaction parameters such as pH, amount of ZnO coating, flow rate and concentration of the dye solution on the percentage removal of dye were examined. In a single pass mode at 30 ml/min flow rate, 52.7% decrease in concentration was achieved for 200muM dye solution (pH 10). It has been demonstrated that in continuous circulation mode the time required to decompose half the concentration of the dye in 500 ml of 200 muM was 15.8 min. Complete removal of 200 muM dye solution (pH 10) was achieved at about 100 min. (C) 2003 Elsevier Science Ltd. All rights reserved.