In2S3 thin films were grown on glass substrates by means of the vacuum thermal evaporation technique and subsequently thermally annealed in nitrogen and free air atmosphere from 250 to 350 degrees C for different durations. Experimental parameters have been adjusted in order to optimize the annealing conditions, and to obtain high band gap energy at low deposition temperature, as required for photovoltaic applications. In order to improve our understanding of the influence of the deposition and annealing parameters on device performance, we have investigated our indium sulfide material by X-ray diffraction, energy dispersive X-ray analysis (EDAX), atomic force microscopy (AFM) and spectrophotometry. The optical and structural properties of the films were studied as a function of the annealing temperature and durations. X-ray diffraction analysis shows the initial amorphous nature of deposited In-S thin films and the phase transition into crystalline In2S3 upon thermal annealing. Films show a good homogeneity and optical direct band gap energy about 2.2 eV. An annealing temperature of 350 degrees C during 60 min in air atmosphere were the optimal conditions. (c) 2006 Elsevier B.V. All rights reserved.