The dispersive refractive index n(lambda) and extinction coefficient k(lambda) of CdS thin films were evaluated from spectrophotometric transmittance characteristic in the visible region. A method is presented to determine n(lambda) and k(lambda) in the region k2 much less than n2, using wavelength and transmittance values. This method has been used for the investigation of the effect of the deposition rate on the optical properties of cadmium sulfide films. The refractive indices are found to depend on the deposition Tate with values between 2.48 and 2.60. The energy gap of the samples is found to be independent of deposition rate with the value 2.44 eV. The real and imaginary parts of the dielectric constant have also been evaluated.