The precipitation kinetics of the metastable calcium sulfate hemihydrate (HH) from aqueous solutions has seldom been addressed. For the first time, the nucleation kinetics of calcium sulfate (probably in alpha-HH form) in concentrated CaCl(2) solutions at 90 degrees C and the effect of Mg(2+) ions were investigated by spontaneous precipitation experiments and a turbidity monitoring method. The results indicate that, at a moderate supersaturation with respect to calcium sulfate, Mg(2+) ions retard the nucleation with 0.01 M Mg(2+) ions showing the strongest effect; at a higher supersaturation, the inhibition effect decreases when the concentration of Mg(2+) ions increases from 0.001 to 0.10 M, and 0.20 M Mg(2+) ions show a promoting effect on the precipitation. The interfacial tension values increase with the concentration of Mg(2+) ions, resulting in a decrease in nucleation rate in Mg(2+) ion-containing solutions compared to Mg(2+) ion-free solutions. This may indicate an involvement of Mg(2+) ions in the surface reaction during calcium sulfate precipitation.