A series of mixed crystals composed of Cu₂ZnSnS₄, Ag₂ZnSnS₄ and ZnS was prepared by co-precipitation of the corresponding metal ions in aqueous sodium sulfide followed by annealing in a sulfur atmosphere. Ideal solid solutions of Cu₂ZnSnS₄ and Ag₂ZnSnS₄ with a kesterite structure ((Cu_xAg_1−x)₂ZnSnS₄ (0 ≤ x ≤ 1)) were successfully obtained by this procedure, as confirmed by their X-ray diffraction (XRD) patterns and energy-diffuse X-ray (EDX) analyses. On the other hand, the solubility of ZnS in these kesterite compounds was found to be limited: the upper limit of the ratio of ZnS to (Cu_xAg_1−x)₂ZnSnS₄ was less than 0.1, regardless of the Cu–Ag ratio in (Cu_xAg_1−x)₂ZnSnS₄. Based on the results for dependence of their photoabsorption properties on atomic compositions, a plausible band structure is discussed. Evaluation of the photocatalytic activity for H₂ evolution of these mixed crystals from an aqueous solution containing S²⁻ and SO₃²⁻ ions upon loading Ru catalysts under simulated solar radiation (AM 1.5) revealed that active compounds for this reaction should contain both dissolved ZnS and Ag components. The dissolved ZnS in (Cu_xAg_1−x)₂ZnSnS₄ gave upward shifts of their conduction band edges. Moreover, the presence of Ag in the solid solution provided n-type conductivity, leading to efficient migration of photogenerated electrons to the surface to induce water reduction into H₂.