Using the chemical bath deposition (CBD) method, CdS thin films in the cubic metastable phase were grown on glass substrates. On thermal annealing (TA) in Ar+S-2 atmosphere in the 240-510 degrees C temperature range, the samples transform to the wurtzite phase. This hexagonal crystalline phase is the stable phase for CdS. From X-ray diffraction analysis the transition point from zincblende to wurtzite is found to occur in the 240-300 degrees C region. From photoacoustic absorption spectra the energy band gap E(g) of the semiconductor was calculated. A plot of E(g) vs. TA shows a minimum value (2.28 eV) at 300 degrees C, in the region of the transition point. Photoluminescence measurements, in the 1.40-2.60 eV emission energy range, show the ’well known’ green emission band for the as-grown sample and for those annealed at high temperatures (360-510 degrees C). For samples annealed in the intermediate region (240 and 300 degrees C), a second band located at 2.2 eV is present. This band, denominated the yellow band and arising in the region of temperature of the transition point, is ascribed to Cd interstitials. A mechanism relating the Cd interstitials to the phase transformation is proposed.