The flower-like In3+-doped SnS2 (InSnS2) was synthesized by microwave-assisted method for the photoreduction of toxic Cr(VI) to less-toxic Cr(III). InSnS2 showed a narrow bandgap (2.0 eV) and a broad spectrum response (400-800 nm), suitable to apply the photocatalytic reduction of Cr(VI) under visible-light irradiation. In our latest report, we found that the enhanced photocatalytic activity was caused by the indium doping and the formic acid, which acted as the inhibitor to hole-electron recombination and hole scavenger, respectively. Herein, as a further study, the contact time of Cr(VI) with InSnS2 in the dark condition was adjusted to ensure the adsorption-desorption equilibrium, and its effect on the reduction activity was evaluated. The results show that spontaneous reduction of Cr(VI) was observed in the initial adsorption process in the dark condition via the synergistic effect of In3+ release and HCOOH, and at later the reduction of Cr(VI) only occurred by the photocatalytic reaction under visible-light irradiation. Finally, the adsorption of Cr(VI) on photocatalyst before light irradiation was found as an important factor to enhance the photoreduction of Cr(VI). (C) 2017 Elsevier B.V. All rights reserved.