The equilibrium temperature for the reaction CaSO*l4*s.2H*l2*sO=CaSO* l4*s+2H*l2*sO*l(liq. soln)*s has been determined as a function of activity of H* l2*sO (a*lH*l2*sO*s) of the solution. Synthetic gypsum and anhydrite or 1:1 mixtures were stirred in solutions of known a*lH*l2*sO*s (calculated from vapor pressure data for the Na*l2*sSO*l4*s and H*l2*sSO*l4*s solutions), at constant temperature for as much as 12 months. The reversible equilibrium was approached from both sides and is defined by: a*lH*l2*sO*s=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 a*lH*l2*sO*s only and is independent of the constituents in solution. Extrapolation to the bounding system CaSO*l4-*sH*l2*sO(a*lH*l2*sO*s=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 sea-water 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.