The ratio of ZnS to AgInS2 is usually adjusted to tune the band gaps of this quaternary (Ag-In-Zn)S semiconductor to increase photocatalytic activity. In this study, the [Zn]/[Ag] ratio was kept constant. The hydrogen production rate was enhanced by increasing the content of indium sulfide. Compared to the steady H-2 evolution rate obtained with equal moles of indium and silver ([In]/[Ag] = 1, 0.64 L/m(2) h), that obtained with In-rich photocatalyst ([In]/[Ag] = 2, 3.75 L/m(2) h) is over 5.86 times higher. The number of nanostep structures, on which the Pt cocatalysts were loaded by photodeposition, increased with the content of indium. The indium-rich samples did not induce phase separation between AgxInxZnyS2x+y and AgIn5S8, instead forming a single-phase solid solution. Although the photocatalytic activity decreased slightly for bare In-rich photocatalysts, Pt loading played a critical role in the hydrogen production rate. This study demonstrates the significant effect of In2S3 on this unique (Ag-In-Zn)S photocatalyst Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.