The paper describes studies on the compositional, structural and optical characteristics of indium sulfide films deposited by thermal evaporation of chemically synthesized indium sulfide powders and subsequently annealed in vacuum in 523-773 K temperature range. Rutherford backscattering spectrometry measurements and depth profiling of sulfur by S-32(p, p'gamma)S-32 nuclear resonance reaction analysis show that the pristine films possess the nominal atomic composition of indium sulfide (In2S3) which remains invariant during vacuum annealing. The pristine as well as vacuum annealed films do not possess, as measured by O-16(alpha, alpha)O-16 resonant scattering, any detectable oxygen contamination. The pristine films are amorphous but crystallize into beta-In2S3 (tetragonal) phase on annealing. The pristine films and those annealed at high temperatures (>673 K) exhibit about 65% transmission in visible region. These films are characterized by both indirect and direct band gaps which increase with increase in annealing temperature. The band gaps lie in 1.9-2.2 eV and 2.6-3.3 eV regions respectively. The electrical resistivity of the pristine films is about 5x10(5) Omega.cm which decreases by about an order of magnitude on annealing in vacuum in the 523-623 K region. It, however, increases with subsequent increase in annealing temperature. The structural and the physical properties of the films are explained in terms of the defective spinel structure of beta-In2S3 and the inclusion of Na (from the glass substrate) into the lattice of In2S3 at higher annealing temperatures. (C) 2012 Elsevier B. V. All rights reserved.