Cd_1-xZn_xS thin films have been studied extensively as window layers for solar cell applications. However, a mismatch between the Cd_1-xZn_xS and copper-indium-gallium-selenide absorber layers increases with Zn film concentration, which reduces the device efficiency. In this work, Cd_1-xZn_xS thin films with low Zn concentrations were analyzed. The effect of the addition of different molar Zn concentrations to the reaction mixture on the growth mechanism of Cd_1-xZn_xS thin films and the influence of these mechanisms on structural, optical and morphological properties of the films has been studied. Cd_1-xZn_xS thin films were synthesized by chemical bath deposition using an ammonia-free alkaline solution. Microstructural analysis by X-ray diffraction showed that all deposited films grew with hexagonal structure and crystallite sizes decreased as the Zn concentration in the film increased. Optical measurements indicated a high optical transmission between 75% and 90% for wavelengths above the absorption edge. Band gap value increased from 2.48 eV to 2.62 eV, and the refractive index values for Cd_1-xZn_xS thin films decreased as the Zn increased. These changes in films and properties are related to a modification in growth mechanism of the Cd_1-xZn_xS thin films, with the influence of Zn(OH)₂ formation being more important as Zn in solution increases.