CdS was deposited on a glass substrate by photochemical deposition (PCD). The substrate was held in an aqueous solution containing S2O32- and Cd2+ and was irradiated with a mercury lamp. Then a CdS film was deposited in the irradiated region of the substrate. The deposition rate depended on various experimental parameters, such as pH of the solution, light intensity, and stirring speed, which were investigated. Here we discuss the deposition mechanism. It was shown that the deposition rate is determined by (i) density of nucleation sites on the substrate, (ii) light intensity, and (iii) transport of the ions in the solution. In the stirred solution, the former two factors are dominant. It was also demonstrated that a pattern can be made on the film using a mask. This proves that PCD has good controllability and thus is well suited for fabrication of devices such as solar cells.