Pt-loaded ZnS-CuInS2-AgInS2 solid solutions showed photocatalytic activities for H2 evolution from aqueous solutions containing SO32- and S2- as sacrificial reagents under visible light irradiation (λ ≥ 420 nm). The crystal structures of the solid solutions were either zinc blende or wurtzite, depending on the composition. The diffuse reflectance and photoluminescence spectra of the solid solutions were shifted monotonically to longer wavelengths as the ratio of MInS2 (M = Cu and Ag) to ZnS increased, indicating that the energy band structure of the sold solution was controllable by the change in the composition. The ZnS-CuInS2-AgInS2 solid solutions exhibited absorption bands that were longer in wavelength than those for the ZnS-CuInS2 and ZnS-AgInS2 solid solutions, which is probably due to interactions between the Cu 3d and Ag 4d orbitals. The photocatalytic activity for H2 evolution was much improved by low-temperature synthesis (773-873 K) and the loading of Ru cocatalysts. The Ru-loaded Cu0.25-Ag0.25In0.5ZnS2 solid solution, which had a wide absorption band (band gap, 1.77 eV) in the visible light region, showed high activity for H2 production, even under simulated solar irradiation (AM 1.5).