Gypsum (CaSO$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$·2H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O) is a hydrated mineral containing crystallization water, also known as gypsum hydration water (GHW). We determined isotope fractionation factors (α$\mathrm{<mrow\; data-mjx-texclass="ORD">17</mrow>}$O, α$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O and αD) between GHW and free water of the mother solution in the temperature range from 3 °C to 55 °C at different salinities and precipitation rates. The hydrogen isotope fractionation factor (αD) increases by 0.0001 units per °C between 3 °C and 55 °C and salinities <150 g/L of NaCl. The αD is 0.9812 ± 0.0007 at 20 °C, which is in good agreement with previous estimates of 0.981 ± 0.001 at the same temperature. The α$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O slightly decreases with temperature by 0.00001 per °C, which is not significant over much of the temperature range considered for paleoclimate applications. Between 3 °C and 55 °C, α$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O averages 1.0035 ± 0.0002. This value is more precise than that reported previously (e.g. 1.0041 ± 0.0004 at 25 °C) and lower than the commonly accepted value of 1.004. We found that NaCl concentrations below 150 g/L do not significantly affect α$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O, but αDgypsum-water increases linearly with NaCl concentrations even at relatively low salinities, suggesting a salt correction is necessary for gypsum formed from brines. Unlike oxygen isotopes, the αD is affected by kinetic effects that increase with gypsum precipitation rate. As expected, the relationship of the fractionation factors for $\mathrm{<mrow\; data-mjx-texclass="ORD">17</mrow>}$O and$^{18}$8Ofollowsthetheoreticalmass-dependentfractionationonEarth($\textit{θ}$ = 0.529 ± 0.001). We provide specific examples of the importance of using the revised fractionation factors when calculating the isotopic composition of the fluids.