This paper reports the preparation of (AgIn)xZn2(1-x)S2 solid solutions from acidic aqueous solutions consisting of silver nitrate, indium nitrate, zinc nitrate, and thioacetamide, without using toxic H2S gas. The reaction sequence of this multicomponent material was investigated, as well as the post thermal treatment conditions. The formation of quaternary semiconductor solid solutions after 800 °C annealing were determined by X-ray diffraction. The peaks in the XRD patterns shifted to higher angles with an increase of [Zn]/[In] in the precursor solutions. Band gaps of the (AgIn)xZn2(1-x)S2 solid solutions were estimated to be 2.11-2.45 eV by diffuse reflection UV-Vis spectra. In addition, the band gap energy increased with [Zn]/[In] accordingly. The precipitates after heat treatment had a layered structure, as revealed by SEM; the compositions were analyzed by EDS. The photocatalytic H2 evolution rates in sacrificial reagents under a 300 W Xe lamp of Pt-loaded (AgIn)xZn2(1-x)S2 solid solutions were measured. A H2 evolution of 340 μmol/h for 0.5 g photocatalyst was obtained. The particles retained the photocatalytic activity for at least 100 h. This study demonstrates that AgInS2-ZnS solid solution particles can be prepared from a completely aqueous route.