A circulating fluidized bed (CFB) system of photocatalyst is studied toward successive treatment of large amount of gas such as tunnel exhaust. The experimental system is 2 m high. A riser of the CFB is concentric double-pipe structure. A light source is installed inside the inner pipe. Silica gel powder coated with titanium dioxide by the sol-gel method provides composite particles of the photocatalyst. Average size of composites is 203 mu m. Composite particles are fluidized and carried upward in the annulus area sandwiched between the inner and outer pipes. An endurance test of a photocatalyst composite is carried out for attrition using the CFB for long time operation. An observation by SEW EDX revealed that a part of photocatalysts on the surface of the composites falls off at the initial stage of the solid circulation and others adhere to the composites tightly. In addition, the photocatalytic removal of NOx is also studied by measuring NO and NO2 concentration. The NO, removal efficiency depends on the solids volume fraction of photocatalyst composites in the riser of the CFB. The mechanism of the NO, removal by the photocatalyst composite in the CFB is thought to be divided into oxidation and adsorption processes. The adsorption process is regarded as more important than the oxidation processes in this experimental system. (c) 2004 Elsevier B.V. All rights reserved.