Adsorption-desorption of ammonia on cadmium sulfate (3CdSO(4). 8H(2)O) has been studied. Its role in formation of cubic phase of CdS when cadmium sulfate is reacted with H2S has been investigated in detail using X-ray diffraction, electron diffraction, infrared spectroscopy, temperature-programmed desorption of NH3, and thermogravimetric analysis. Three distinct; acidic sites on cadmium sulfate where ammonia can get chemisorbed have been reported. The heats of adsorption for the three sites are 24.3, 26.9, and 42.6 kcal mol(-1), respectively. The origin of surface acidity is related to the formation of transition intermediates during the dehydration of 3CdSO(4). 8H(2)O. Both Bronsted and Lewis acidities have been observed. The reaction of ammonia preadsorbed cadmium sulfate with H2S gas leads not only to the formation of larger crystallites but also to the cubic phase of CdS which has lower photocatalytic activity for hydrogen production from water. Ammonia can be completely desorbed from the surface of cadmium sulfate if it is heated above 600 K.