In2S3 colloidal thin films with various thickness were prepared by the spin coating method. The band gap of the thin films depended on the thickness and decreased with heat treatment. The dependence of the band gap on the thickness and heat treatment was explained well by a size quantization effect of In2S3 colloidal particles in the him. In2S3 colloidal thin films prepared on wide gap semiconductors such as TiO2 and ZnO showed an anodic photocurrent under visible light irradiation. The mechanism of semiconductor sensitization and efficiencies of the charge injection were studied based on the photoelectrochemical properties and action spectra of In2S3\TiO2, In2S3\ZnO and In2S3\ZnS photoelectrodes.