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Infrared spectra of the materials composing the $CaS{O}_4\cdot 2H_{2}O-CaS{O}_4$ system have been examined. $CaS{O}_4\cdot 2H_{2}O$ films laid down from a n-hexane suspension on KBr disks were heated to controlled temperatures in the atmosphere, under vacuum ($5\cdot {10}^{-2}$mmHg) and at the vapour pressure of 21 mmHg. Infrared spectra indicate that a direct transformation of $CaS{O}_4\cdot 2H_{2}O$ into anhydrous $CaS{O}_4$ occurs in a definite temperature interval, which depends on each case. Infrared spectra, recorded from $CaS{O}_4\cdot 2H_{2}O$ film heated under the vapour pressure of 140 mmHg, show, on the contrary,...