The activity and selectivity of platinum-modified zeolites (Beta, Y, Mordenite) were investigated in the ring opening of decalin at 473-543 K in the presence of hydrogen. In the course of decalin transformation, skeletal isomerization, stereoisomerization, ring opening, and cracking took place. The presence of platinum resulted in an enhancement of isomerization and ring-opening rates, as compared to the proton-form zeolites. The isomerization and ring-opening rates increased 3 and 5 times, respectively. Hydrogen pressure was found to suppress the secondary reactions and to prevent the catalyst deactivation. The isomerization and ring-opening reactions were not affected by hydrogen presence. Variations in catalyst deactivation were observed and attributed to different locations of organic deposits. Interactions between platinum and Brønsted acid sites suggested that the platinum crystallites were located partially in the channels of the studied zeolites. As a result of these interactions, the strength of Brønsted acid sites was reduced and consequently less cracking products were formed, at comparable conversions, over platinum-modified zeolites than over the corresponding parent zeolites.