Thin films of Bi2S3 with different thicknesses were prepared by the chemical deposition method from an aqueous acidic bath using thiosulfate as a sulfide ion source. The effect of film thickness on the optical, structural and electrical properties was studied. A shift of 0.6 eV in the optical bandgap energy, E-g, and a decrease in electrical resistivity from 2.8 x 10(4) to 5 x 10(3) Ohm cm and an increase in grain size of Bi2S3 crystallites from 5.2 to 8.0 nm were observed when the thickness was varied from 52.7 to 220 nm. These changes are attributed to the quantum size effect in semiconducting films.