In Fig. , we compare the experimental solubility values of gypsum and bassanite in a 4.8 M NaCl brine at different temperatures, as extracted from the Raman spectra, with the solubility curves calculated using PHREEQC 3 (see also Fig. ). This graph reveals a notable discrepancy between the theoretical curves and the experimental data. At room temperature, the measured solubilities of gypsum and, especially, bassanite are lower than the calculated solubilities. This finding is remarkable because solubility measured from precipitation experiments typically overestimates the actual solubility due to the time required to reach true equilibrium. Noteworthy, when the temperature is rapidly increased to T = 90 °C, the observed increase in free sulfate concentration in the solution contradicts expectations, as it was commonly assumed that the solubility of gypsum significantly decreases with rising temperatures under low salinity conditions 1 . However, the revised PHREEQC 3 code predicts this increase in gypsum solubility with temperature for highly saline solutions. Overall, numerical solubility predictions in brines are (still) not accurate enough to determine the phase stability in the CaSO 4 system.