The presentation deals with the effect of high temperatures on the properties of gypsum paste (prepared by mixing water and calcium sulfate hemihydrate) and gypsum mortar (gypsum paste + fine aggregate). The experiments were focused on the behavior of the gypsum paste and the gypsum mortar during heating to temperatures up to 1300 °C and after cooling to room temperature. The behavior of the gypsum paste during heating was monitored by thermal analysis (including dilatometry) and X-ray diffraction (XRD) analysis with a heated chamber. The structural changes of the gypsum paste and the gypsum mortar after cooling were tested by electron microscopy, X-ray diffraction analysis (XRD) and mercury porosimetry. Furthermore, the changes in the pastes and mortars' basic physical and mechanical properties and the ability to transport water vapor and moisture were monitored. Interesting knowledge about the existence of the individual phases (gypsum, calcium sulfate hemihydrate and forms of anhydrite AIII and anhydrite AII), about the microstructure (cracking of crystals during dehydration; fragmentation and sintering of anhydrite AII at temperatures above 700 °C, etc.) and about their external consequences (changes in strength, shrinkage) has been gained. The gypsum mortar had better volume stability during heating (its volume decreased at 1000 °C by approx. 10%) compared to the gypsum paste (its volume decreased by approx. 50% at 1000 °C), but worse mechanical properties due to the interfacial transition zone failure.