Due to its importance as a perspective material for application in optoelectronic semiconductor devices, the thermal annealing effects on the structural and optical properties of the as-grown vacuum evaporated AgIn5S8 thin films have been investigated. The X-ray data analysis have shown that these films are polycrystalline in nature and exhibit better crystallization with increasing crystallite size and slightly, decreasing unit cell lattice parameter as annealing temperature is raised from 450 to 600 K. The optical energy band gap for the as-grown and thermally annealed films is found to be of direct allowed transitions type. The energy band gap exhibited values of 1.78, 1.74 and 1.62 eV as the samples were annealed at, 450 and 600 K, respectively. This indicates the ability of altering the band gap values of this material by the thermal annealing process. The structural and optical features seem to be suitable for semiconductor device production such as solar cell converters, which has successfully been fabricated by others, metal-insulator-semiconductor (MIS) and metal - oxide - semiconductor (MOS) devices, as well. (c) 2007 Elsevier B.V. All rights reserved.