The metastable nature of molecular-beam epitaxy grown CdS layers was studied. Undoped CdS layers were grown on (100) GaAs substrates/(100) ZnSe buffer layers, and crystal structures were characterized by x-ray diffraction and transmission electron microscopy. Undoped films showed both zincblende and wurtzite structures depending on the growth condition; higher substrate temperatures and smaller VI/VII beam intensity ratios preferentially form wurtzite structures. The Ga doping significantly affected the crystal structure and Ga doped CdS layers preferentially formed wurtzite structures. Thermal instability of the CdS him was observed through various ex situ annealing experiments. The wurtzite structure phase was developed from the zincblende structure CdS layers by thermal annealing where the annealing temperature was lower than the growth temperature.