Thin transparent films of tin-doped cadmium sulfide (CdS) on different substrates were obtained by an ammoniafree chemical bath deposition technique assisted by ultraviolet illumination with a wavelength of 265 nm. Several sets of films were deposited by varying the amount of SnCl2 in the reaction bath and analyzed by X-ray diffraction, scanning electron microscopy, optical transmission and reflection spectroscopy and current versus voltage measurements. In the doped films, the position of the main X-ray diffraction peak is shifted to larger angles showing a decrease of the crystal lattice constant; this is attributed to the substitution of tin by cadmium in the doping process. The morphological characterization of the doped films shows homogeneously distributed spherical grains with an average grain size of 58.5 nm. The grains are composed of crystallites with an average size of 23 nm; the surface of the doped films does not show conglomerates that are typical for materials grown by the chemical bath deposition technique. The electrical properties reveal a decrement in the resistivity of an order of magnitude with respect to the films without doping. The optical results exhibit variation in the band gap that we consider as the consequence of the films' porosity; the theoretical analysis presented confirms this point. The films show a reflectance at a normal incidence less than 30%, which is an indication of their good optical quality. The main effects of the tin doping on the CdS films were observed as changes in the crystal structure and electrical resistivity.