The equilibrium temperature for the reaction CaSO4·2H2O = CaSO4+2H2O(liq. soln) has been determined as a function of activity of H2O (aH2O) of the solution. Synthetic gypsum and anhydrite or 1:1 mixtures were stirred in solutions of known aH2O (calculated from vapor pressure data for the Na2SO4 and H2S04 solutions), at constant temperature for as much as 12 months. The reversible equilibrium was approached from both sides and is defined by: aH2O = 0.960 at 55°, 0.845 at 39°, 0.770 at 23°C. Provided the solids do not change in composition, the equilibrium at constant P and T is a function of only and is independent of the constituents in solution. Extrapolation to the bounding system CaSO4-H20 (aH2O = 1.000) yields 58° ± 2°C. This is within thermodynamic calculations (46° ± 22°C) but higher than solubility measurements (38° to 42°C). The new data indicate that in seawater saturated with halite and gypsum should dehydrate above 18°C. The scarcity of anhydrite in modern evaporite deposits is predicted by the present results. The available data on the temperature-salinity conditions under which anhydrite and gypsum exist in the Recent supratidal flat sediments of the Trucial Coast, Persian Gulf, are compatible with the present experimental data.