Electronic redox levels of Ag clusters in an aqueous solution and at the solid-aqueous solution interface have been estimated by using the numerical data of Mitchell about the electronic properties of Ag clusters in a vacuum. We have compared these levels with both the standard redox potentials of H₂O, alcohols (CH₃OH, CH₃CH₂OH, (CH₃) 2CHOH), a sensitizing dye (Uranine) and the energy bands of solids ( n -ZnO, n -TiO₂, SiO₂) in order to clarify the nature of the photoreduction of Ag⁺. The quantum yield of the photoreduction has changed markedly with different reaction conditions such as exposure of the reaction system to UV-light absorbed by Ag⁺, to near UV-light absorbed by the solid, and to visible light absorbed by dye molecule. The existence of the semiconductor surface could stabilize the energy levels of the Ag clusters by adsorption and promote the photochemical electron-transf er reaction at the solid-aqueous solution interface.