Abstract: Hydrothermal anhydrite [CaSO4] is a good host for primary and secondary fluid inclusions. It is also potentially a problematic host for stretching of inclusions, partial dissolution and precipitation of anhydrite, and formation of gypsum from reaction of anhydrite with the trapped aqueous fluid. Microthermometric experiments on liquid/vapor inclusions in anhydrite from Juan de Fuca Ridge show that new daughter crystals, identified to be the low-temperature hydrate gypsum [CaSO4?2H2O], commonly develop during freezing experiments (~-50℃).This phenomenon has been recognized for over 20 years, but its occurrence and significance to the determination of inclusion salinities are not widely appreciated. There is growing evidence that hydration of the host anhydrite may significantly lower the ice melting temperature and increase the salinity of the residual inclusion fluid. The present study shows that the salinities of inclusions increase largely after gypsum formed in liquid/vapor inclusions (salinities increase 0.32 to 1.6% NaCl eq.). We propose that gypsum is the product of both anhydrite hydration and supersaturation with respect to gypsum, but mainly owing to the former. Heating inclusions to decompose gypsum results in the release of bound water, restoring the inclusions to their original salinities. So when gypsum formation occurs during freezing experiments, it needs to be dehydrated periodically through heating to about 130℃. These processes could have no impacts on the real homogenization temperatures of inclusions.