Bi2S3 thin films were prepared on amorphous glass substrates by successive ionic layer adsorption and reaction (SILAR) method at room temperature using bismuth nitrate and thioacetamide as the cationic and anionic precursors in aqueous medium. The X-ray diffraction study reveals that as-deposited films of Bi2S3 are amorphous in nature, it becomes polycrystalline after annealing at 573 K. The decrease in activation energy from 0.65 to 0.36 eV and optical band gap energy, E-g, from 2.35 to 1.86 eV are observed as film thickness varies from 67 to 150 nm. Such changes are attributed to the quantum size effect in semiconducting films. (C) 2008 Elsevier B.V. All rights reserved.