Titanium oxide has been developed which can be put to practical use in photocatalysis, and trials have been carried out for the application to environmental processing. Some of the cases are quoted to investigate means for the application of the photocatalysis study. During the process of improving the light resistance of titanium oxide pigments, various knowledge on photocatalytic phenomena has been obtained, and titanium oxide having a strong enough activity suitable for practical use is obtained based on the knowledge. There are two methods for the application of photocatalysis to decomposition and deodorization of malodor in living space. Fixation of NOx has been studied to solve the problem of air pollution. A huge reactor is required if photocatalysis is applied to the treatment of a large quantity of water. Various kinds of approach may be needed to structure a compact reactor suitable for practical use. 14 refs., 3 figs., 2 tabs.