Temperature-programmed desorption of ammonia from the H form of a modernite was carried out after pretreatment at different temperatures. It could be shown that a quantitative registration of the acidic OH groups is possible. Moreover, from the desorption curves, information is obtained about the alteration of the acidic properties as a function of the temperature of pretreatment.H mordenites are frequently used as catalysts because of their porous structure, acidity'and thermal stability [1,2]: In order to characterize the number, nature and strength of the acidic sites ofzeolites, NH3 can be used as a suitable probe molecule. For this, temperature-programmed desorption (TPD) is a suitable method [2-13], besides the microcalorimetric estimation of adsorption heats [14][15][16][17][18][19] and other methods [20][21][22][23]. In this connection, many papers report the change in NH 3 desorption depending on the Si/A1 ratio, and also the dealumination of H mordenites [2,[8][9][10]. The acidic properties ofH mordenites after exchange with K + ions [8], or after treatment with Si(OCH3) 4 [11], are also characterized by this method. However, only limited information is available about the changes in the TPD curves of ammonia after pretreatment at different temperatures. Thus, in the present paper the influence of the temperature of pretreatment of H mordenite on the TPD of ammonia is investigated systematically, because it is known that the adsorption behaviour Of ammonia is changed significantly by reason of the partial dehydroxylation of the zeolite.