We have developed and put in practice a spectrophotometric procedure for hydrogen sulfide determination to study the phase diagram of the In-S system using hydrogen as an auxiliary component. Elemental analysis of the hydrogen-containing vapor phase allows the principal thermodynamic properties of the indium sulfides in equilibrium with the vapor phase to be assessed in wide temperature and composition ranges. Our results confirm the existence of a narrow two-phase field between In3-xS4 and In2S3' (low-temperature phase), which is bounded from above by the peritectic decomposition temperature of In2S3' (about 415 degrees C). We have determined the temperature-dependent sulfur (S-2) vapor pressure over condensed indium sulfides. For the equilibria studied in greatest detail, the present results correlate well with mass spectrometry data for indium sulfides. (C) 2013 Elsevier B.V. All rights reserved.