Cadmium Zinc Sulfide, Cd_1-xZn_xS thin films were deposited by chemical bath deposition technique at bath temperature of 75°C. The morphology of the films was analyzed by scanning electron microscope, the optical constants of the films were estimated from the transmission and reflection spectra of the films in the wavelength range of 300 - 900 nm. The films had a transmittance between 75% and 85% and optical band gap in the range 2.8 - 3.4 eV. The dependence of the refractive index of the films on the wavelength was investigated using the single oscillator model, from which the dispersion parameters were determined. The high frequency dielectric constant ε_L and the ratio of the carrier concentration to the effective mass N/m^* were estimated based on the Spitzer and Fan model. Both ε_L and N/m^* show a decrease in value with increase in Zinc content.