Hydration enthalpies of synthetic A-type and some natural zeolites have been measured by an adiabatic gas-absorption calorimeter to evaluate zeolite as a heat exchange absorbent for a solar-powered system. The Mg-exchanged A-zeolite is a suitable absorbent, if it is dehydrated at 100 degrees C. The total hydration heat is the highest (531 kJ kg(-1)) of the A-type zeolites treated in the present work and is twice that of natural mordenite or clinoptilolite. An empirical equation is proposed to simulate the relation between differential enthalpies and hydration amounts.