In recent years, In2S3 has been used as a buffer and/or window layer in photovoltaic solar cells due to its high optical energy band gap and controllable physical properties. The usage of different materials in a p-i-n solar cell leads to mismatch of various parameters at the interfaces, thereby increasing the recombination current. Hence, it is more advantageous if a single material is used as both i- and n-layers in solar cells, whose properties can be altered by simple chemical and/or heat treatment. In this work, phase conversion of In2S3 into In2O3 was investigated by annealing close spaced evaporated In2S3 films in air. The effect of annealing temperatures on composition and optical properties of the layers are studied. In2S3 films were prepared by close spaced evaporation of In2S3 powder. The layers were grown on glass slides at a substrate temperature of 350oC. The source-substrate distance was maintained as 5 cm with the rate of deposition as 30 Å/sec. The thickness of the layers was ~300 nm. The layers were annealed in air at different temperatures that vary from 250oC to 400oC for 1 hr. Chemical composition of the films was determined using Energy Dispersive Analysis of X-rays (EDAX). The optical properties, transmittance and reflectance of the films were analyzed using Pye Unicam UVVisNIR spectrophotometer and other optical parameters such as absorption coefficient, energy band gap, refractive index, extinction coefficient etc. were determined. The as-grown In2S3 films appear in pale yellow colour and changed to pale whitish in appearance on annealing in air. Fig.1 shows the EDAX spectrum of completely converted In2O3 film. All the annealed films showed the presence of peaks that correspond to In, S and O. No other impurity was found in the film within the EDAX detection limit. The O content in the films increases with annealing temperature (Ta) whereas S content in the film decreases. This is probably due to the reevaporation of S from the film during the post-deposition annealing because of its high vapour pressure, forming S vacancies. These S defects were subsequently filled by O atoms from air by diffusion from surface into the bulk of the films as the formation energy of In-O bond is lower than that of In-S bond. The optical analysis of the layers indicated an optical transmittance > 70% for all the layers and it increased with the increase of annealing temperature. The evaluated energy band gap of the films varied in the range, 2.35 - 2.62 eV with increase of annealing temperature. This increase of energy band gap with temperature might be due to replacement of S by O atoms in the film as the former might have re-evaporated from the film owing to its high vapour pressure at high temperatures. The other optical parameters such as refractive index and extinction coefficient were also determined.figureFig.1. EDAX spectrum In2S3 film. Fig.2. Transmittance versus wavelength spectrum.