WO₃ is a visible-light–responsive n-type semiconductor photocatalyst, but reports on the degradation of organic substances over WO₃ are limited because of its low photocatalytic activity towards organic substances. However, the activity can be enhanced by loading co-catalyst or utilizing redox mediator. In this study, we tried to utilize several transition metal ions to promote the photodegradation activity over WO₃, and found that ferric and cupric ions were suitable to promote the reaction. We estimated the activity of WO₃ in the photodegradation of organic substances by the amount of CO₂ generated in the reaction, and it was determined using a gas chromatograph equipped with a methanizer. For ferric ion, the photoexcited electron of WO₃ reduced it to ferrous ion instead of oxygen reduction reaction. This reaction promoted the charge separation and enhanced the photodegradation activity. However, ferric ion worked sacrificially because the oxidation rate of ferrous ion by air was pretty low compared with the generation of ferrous ion by the photocatalytic reduction in the acidic aqueous solution medium. On the other hand, cupric ion could work cyclically as redox mediator. Cupric ion was reduced to cuprous ion by photoexcited electron from WO₃ and cuprous ion easily oxidized to cupric ion again by oxygen in air. This redox cycle could assist the reduction reaction of oxygen in photodegradation of organic substances. In our presentation, we will discuss the details of the reaction and show the suitable systems to utilize ferric and cupric ion, respectively.