Crystallization of calcium sulfate dihydrate (CaSO4 center dot 2H(2)O, gypsum) in sodium chloride solutions has been studied in the presence of magnesium ion. It was shown that Mg2+ has an inhibiting effect on the two stages of precipitation: nucleation and growth. Kinetic results were explained on the basis of the strong effect of magnesium ions on gypsum solubility. In the studied range of the ionic strength (IS), it was found that in the presence of NaCl the solubility increase stops beyond IS = 0.31 mol.L-1 whereas it continues to rise in the presence of MgCl2. X-ray diffraction analyses showed that magnesium ions act by substituting the calcium ones or by insertion in the defects of the gypsum crystal lattice.