The contribution of free electrons to the refractive index and extinction coefficient of Ga‐doped CdS has been measured in a series of samples with carrier concentrations ranging from about 1017to 2×1019electrons/cm3. The effective massm* for electrons near the bottom of the conduction band was calculated from the free electron contribution to the refractive index and from the carrier concentration as determined from the Hall coefficient. The result,m*=(0.22±0.01)meis in satisfactory agreement with previous studies by other workers. The magnitude and wavelength variation of the absorption coefficient observed with about 1017carriers/cm3are in fair agreement with theoretical results calculated for a polar‐mode lattice scattering mechanism. Although similar data for 2×1018carriers/cm3are in good quantitative agreement with the impurity scattering theory at room temperature, the absorption for high carrier concentration is observed to decrease with temperature, in contradiction with the theory for a nondegenerate population. A theory for the degenerate concentration range is needed. Reflection and transmission of radiation polarized parallel and perpendicular to the crystalCaxis were studied for one sample. These data show (m⊥*/m∥*)=1.08±0.05.