The activity of H-Beta-25, H-Beta-75, H-Y-12, H-Mordenite-20, and H-MCM-41 was investigated in the ring opening of decalin at 498-573 K in the presence of hydrogen. The catalysts activity and deactivation were correlated to their acidity determined by FTIR pyridine adsorption, pore structure, and surface area changes. Skeletal isomerization, stereoisomerization, ring opening, and cracking were the prevailing reactions taking place. Skeletal isomerization included several parallel and consecutive reactions resulting in a wide variety of products, which underwent ring opening, and subsequently cracking. Virtually no direct ring opening of decalin was observed; i.e., decalin must first be isomerized to alkyl-substituted bicyclononanes and bicyclooctanes to make possible its ring opening. The main products were grouped according to their structural features to evaluate the product distribution, which was influenced by catalyst acidity and pore structure. The initial activity of the catalysts increased with increasing acidity and reaction temperature, respectively. The deactivation of the investigated catalysts depended on their pore structure; H-Y was deactivated faster than Beta zeolites as it allowed the formation of large hydrocarbon intermediates inside the cavities. (C) 2003 Elsevier Inc. All rights reserved.