Photocatalytic reactors are usually operated by suspending photocatalyst particles or packing with photocatalyst immobilized on any solid support, which provide heterogeneous reaction systems where reactions take place on the two-dimensional plane. Thus, these conventional reactors have unavoidable limitation to enhancement in the reaction rate because it is difficult to acquire a sufficient reaction field. On the other hand, titanium dioxide (TiO₂) can be dissolved in an aqueous solution of H₂O₂ to provide a transparent yellow solution, which enables to prepare a transparent TiO₂ thin film on the surface of a solid support. Based on this coating technique, two types of photocatalytic reactors were constructed; 1) consisting of a glass column packed with glass beads or short glass tubes and 2) a circular arrangement of 19 glass tubes linked in series and a fluorescent light positioned in the center. To investigate the performance of these two reactors, photocatalytic degradation of 2, 4-dinitrophenol was carried out in a batchwise recirculation system. Furthermore, the latter reactor was applied to oxidation of ethylene as an example of application to CELSS.