The activity and selectivity of hydroisomerization of n-hexane over Pt/mordenite is strongly influenced by acid leaching. The activity increases and the selectivity changes to favor primary products. Leaching selectively modifies the mordenite structure, making more sites accessible for reaction and facilitating desorption of reaction products. It is argued that the activity of untreated mordenite is limited by mass transfer effects. These effects largely vanish after modification of the zeolite structure by generation of a 3-D micropore structure as well as mesopores. The alleviation of intracrystalline diffusion Limitation is the major factor in activity enhancement after acid leaching of Pt/mordenite. (C) 2000 Academic Press.