The formation of double salts in the system Na2SO4-CaSO4-H2O has been investigated by solid-gas reactions. The hydration processes of the quenched phases 2Na(2)SO(4) . CaSO4 and (Na0.8Ca0.1)(2)SO4 have been monitored by thermogravimetry and X-ray diffraction in the presence of a definite water vapour pressure. Hydration of the metastable phase 2Na(2)SO(4) . CaSO4 leads to the formation of glauberite and thenardite, preceded by an intermediate formation of the ’labile salt’ (2Na(2)SO(4) . CaSO4 . 2H(2)O). The second quenched phase, (Na0.18Ca0.1)(2)SO4, reacts to hydroglauberite (3Na(2)SO(4) . 5CaSO(4) . 6H(2)O) and thenardite, [Na2SO4 (V)].