The construction of calorimetric ampoules for work in hydrothermal conditions makes it possible to investigate the enthalpy of mineral crystallization. The potential of the equipment is demonstrated for the case of zeolite synthesis from glasses, Two synthetic Na- and Na-K-glasses, and dacite pumice were used as starting materials. Aqueous solutions of 2 N and 6 N NaOH and KOH were prepared, The crystallization was performed in a Kalve calorimeter DAC-1A at 95 degrees C. Three peaks are observed in the e.m.f. curves due to : wetting of the glass, glass hydration and dissolution, and crystallization of zeolites. Their position versus time and their intensity depend on the experimental conditions, The described calorimetric equipment is very useful for the low-temperature hydrothermal synthesis of zeolite minerals under isothermal conditions and provides a means of controlling the crystallization process.