CdS polycrystalline thin films were grown by the chemical bath deposition technique at 80 degrees C onto glass substrates. The films grow in the cubic crystalline structure as determined by x-ray diffraction analysis. After thermal annealing in S-2 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 degrees 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 degrees C just before the phase transition takes place. For the sample annealed at 374 degrees 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.