CuIn3S5 compound was prepared by direct reaction of high-purity elemental copper, indium and sulphur. CuIn3S5 thin films were prepared from powder by thermal evaporation under vacuum (10(-6) mbar) onto glass substrates. The glass substrates were heated from 30 to 200 A degrees C. The powder was characterized for their structural and compositional properties by using X-ray diffraction (XRD) and energy dispersive X-ray (EDAX). The XRD studies revealed that the powder exhibiting P-chalcopyrite structure. From the XRD data, we calculated the lattice parameters a and c. Then, the cation-anion bond lengths l (AC) and l (BC) are deduced. The films were characterized for their structural, compositional, morphological and optical properties by using XRD, EDAX, atomic force microscopy and optical measurement techniques (transmittance and reflectance). XRD analysis revealed that the films deposited at a room temperature (30 A degrees C) are amorphous in nature, whereas those deposited on heated substrates (a parts per thousand yen75 A degrees C) were polycrystalline with a preferred orientation along (112) of the chalcopyrite phase. The surface morphological analysis revealed that the films grown at different substrate temperature had an average roughness between 1.1 and 4.8 nm. From the analysis of the transmission and reflection data, the values of direct and indirect band gap of the films were determined. We found that the optical band gap decreases when the substrate temperature increases.