Mirabilite and thenardite are common minerals found in marine evaporite deposits and also in saline lakes as precipitates. In this study, mirabilite-thenardite equilibria were determined along four humidity-buffer curves at 0.1 MPa and between 285.15 K and 305.15 K. Results for the reaction Na(2)SO(4 center dot)10H(2)O((s)) = Na2SO4(s) + 10H(2)O((g)), based on tight reversals along each humidity buffer, can be represented by ln K = 174.032-62815.11/T, where s is solid, g is gas, K is the equilibrium constant and T is temperature in K. Our data are in excellent agreement with several previous vapor-pressure measurements and are consistent with the solubility data reported in the literature. Thermodynamic analysis of these data yields 90.858 kJ.mol (1) for the standard Gibbs free energy of reaction, which is in good agreement with the values of 90.69 kJ.mol 1 and 90.34 kJ.mol (1) calculated from the thermodynamic data compiled and evaluated previously from two sources. (C) 2017 Elsevier Ltd.