Nanocrystalline indium (III) sulfide in film and powder state was synthesized in thermostatic conditions (353 K) by hydrochemical sedimentation from the solution, containing indium (III) salt, tartaric acid, sulfuric acid hydroxylamine and acetothioamide. According to interferention microscopy data, thickness of obtained nanocrystalline layers was 3400±100 nm. X-ray diffraction defined the cubic structure (I41/amd - D194h) of In 2S 3 films with constant crystalline grade, equal to 1.0734 nm. Scanning electron microscopy defined that indium (III) sulphide in powder type consists of spherical aggregates with diameter from 0.3 to 5.5 m, formed by flat crystallites (near 30 nm). Films are formed from fibriform (40-70 nm). Element energy-dispersion microanalysis defined that powder In 2S 3 is lessconcentrated by oxygen (5.9%(at.)), than films' sample (8.0%(at.)), which is connected with larger sizes of powder agglomerates. Elemental analysis of fine-film indium (III) sulfide defined the increasing of oxygen content with thermal annealing in the temperature range of 275-495 оC from 8.5 to 67.8%(at.) with simultaneous decrease of sulfur content from 53.3 to 5.5%(at.). Thermal-gravimetric analysis in the medium of argon defined the stability of powder In 2S 3 to 300 оC to thermal destruction. There was defined the temperature of phase transfer of nanopowder from α-In 2S 3 in -modification, equal to 301 оC. There was made a definition that beginning of thermal-oxidation destruction corresponds to the temperature of 330 оC. Differential-thermal analysis defined that in the process of thermal destruction of In 2S 3 during 330 оC heating, indium (III) sulfide oxidates constantly till 500 оC with formation of indium (III) oxide, sulfur (IV) oxide and indium (III) sulfate. Total loss of mass in samples after heating up to 600 оC was near 14.9%.