Many semiconductor photocatalysts with a narrow band gap, especially those based on TiO₂ species such as TiO_2–xM_x (M = N, C, S, metals), have been extensively investigated with the goal of developing catalysts for the decomposition of harmful organic substances such as volatile organic compounds (VOCs) under visible or fluorescent light for indoor and inside-vehicle applications. However, the ability of these catalysts to completely oxidize organic compounds into CO₂ under visible light has so far been unsatisfactory. WO₃ is a visible-light–responsive photocatalyst and an n-type semiconductor photocatalyst for O₂ generation using sacrificial reagents, but reports on the degradation of organic substances over WO₃ are limited. In addition, there are some practical problems with its use; a precious Pt was used as co-catalyst to increase the activity, and the ability of WO₃ to catalyze complete oxidation into CO₂ was not clearly demonstrated. We previously reported that because of the formation of stable byproducts, acetaldehyde (CH₃CHO) cannot be completely oxidized in the presence of a WO₃ photocatalyst without a co-catalyst under visible light. In the present study, we investigated how loading various metals and metal oxides as co-catalysts on WO₃ photocatalyst affected the oxidation of VOCs such as CH₃CHO under visible and UV light. We found that various Cu-compounds such as CuO and CuBi₂O₄ were very effective co-catalysts over WO₃ for the complete oxidation of various VOCs into CO₂. The reaction mechanism on the loading effect of Cu-compounds was discussed in detail.