Hydrogen and oxygen isotope fractionation factors of water between water of crystallization of carnallite, bischofite and tachyhydrite and the mother solutions were determined from the isotopic activity ratios of the mother solutions (αa) in the temperature range of 10-40°C. Deuterium is depleted and 18O enriched in the water of crystallization of all these minerals. No clear temperature dependence can be observed. The published fractionation factors in the literature, which were all determined from the isotopic concentration ratios of the mother solutions (αc) were corrected to αa using the "salt effect" coefficients. It is proposed that only the αa values can be applied to the natural settings.