CdS polycrystalline thin films were grown by the chemical bath deposition technique at 80 °C onto glass substrates. The films grow in the cubic crystalline structure as determined by x-ray diffraction analysis. After thermal annealing in S2 and Ar atmospheres, the CdS changes from the metastable zinc blende phase to a stable wurtzite one. The cubic-to-hexagonal transition temperature has been determined to be 370 °C, as seen by the photoluminescence spectra and the x-ray diffraction patterns of the different samples. These spectra show the well-known green emission band of the CdS centered at 2.4 eV for the as-grown sample, which shifts to 2.25 eV for the sample annealed at 365 °C just before the phase transition takes place. For the sample annealed at 374 °C, an abrupt blueshift of the green band occurs going back to an energy value of 2.4 eV, when the crystalline phase transition occurs.