In the present study the optical properties of co-evaporated indium sulfide thin films are investigated. Before being optically characterized, the composition as well as the crystalline properties of the film have been checked with the help of energy dispersive spectroscopy (EDX) and X-Ray diffraction (XRD) analyses. The optical absorption coefficient alpha of this indium sulfide film has been deduced from reflectivity R(lambda) and transmission T(lambda) measurements. The fit of the curve representing alpha(hv) suggests that the beta-In(2)S(3) has an indirect bandgap of 2.01 eV. Density functional theory (DFT) calculations are performed on this indium sulfide compound, using TB-LMTO code. Through these band structure investigations, an indirect bandgap is predicted as observed experimentally. The top of the valence band is mainly formed by the orbitals of the sulfur atoms. This observation suggests that an over or under stoichiometry in sulfur may affect both the nature and the width of the indium sulfide bandgap. (C) 2008 Elsevier B.V. All rights reserved.