The undesired structural change of mordenite with Si/Al-2 = 15 during the ion exchange and during calcination from the sodium form into proton form, especially the dealumination after the calcination, was studied. The Si-29 NMR results clarified that calcination of the NH4-form mordenite with Si/Al-2 = 15 at > ca. 800 K, followed by the contact of proton-form sample with the atmosphere containing humidity, induced the dealumination. However, the samples carefully kept free from humidity after the heat treatment showed no dealumination or only a low degree of it. The Al-27 NMR and ammonia TPD results agreed with these findings. The origin of dealumination is thus considered to be the contact of the proton-form sample with humidity after the calcination. On the other hand, the dealumination was not observed after calcination at < 673 K. The H-1 NMR indicated that the ca. 1/3 of the ion-exchange sites were blocked by the ammonium cations after calcination at 673 K, suggesting that the dealumination was prevented by the ammonium cations. On the basis of these findings, a standard set of conditions in which one can safely prepare the proton-form zeolite with high quality is proposed. The recommended temperature for the ion exchange is room temperature, because it is high enough to exchange the ions in this case. The most important variable is the calcination temperature; it should be lower than 673 K in order to avoid the dealumination. These conditions were confirmed to yield zeolites with a high framework aluminum content and ordinary catalytic performance. (c) 2005 Elsevier B.V. All rights reserved.