The stepwise thermodesorption (TD) of NH3 from Na. and H-forms of different types of zeolites (A, Y, mordenite, erionite) was studied by the gravimetric method. The TD-experiments were carried out in the temperature range from 30 to 420 degreesC. A correlation between the temperature of desorption and the heat of adsorption, measured calorimetrically, was discovered. It was shown that thermodesorption data can be used for a rapid estimation of heats of adsorption of NH3 on zeolites and other acid catalysts as well as for determination of number of acid sites.