When TiO2 is immobilized on organic fibres, pumice stone or polymer film, the photocatalytic efficiency decreases slowly during long-term use. The efficiency of immobilized photocatalysts were tested on 500 ml of a 5 x 10(-5) M solution of acid orange-7 (a classical azo dye) before and after treatment of 10(-3) M solution of acid orange-7 during 4 weeks. It was observed that the efficiency was reduced approximately four, five and 10 times with polymer film containing TiO2, TiO2 on organic fibres and TiO2 on pumice stone, respectively, after 4 weeks of use. Volumes treated were 40, 45 and 601, respectively. Nevertheless, the decomposition rate stays a little higher with TiO2 on pumice stone than with the two other catalysts tested. The photocatalytic activity of immobilized TiO2 was significantly reduced also during treated with wastewaters. For immobilized photocatalysts used, the decrease of activity is considered to be caused by the elimination of some particles from the catalyst surface during use and also by fouling of catalyst surface by the formation of by-products during the course of degradation process. (C) 2004 Elsevier B.V. All rights reserved.