Modern combustion engines require fuels with high RON. An ecologically sound way of increasing the RON is by raising the concentration of branched alkanes at the expense of linear alkanes via the (hydro)isomerization of linear alkanes to branched ones. Hydroisomerization requires a bifunctional catalyst consisting of a (de)hydrogenating metal function. Two commercial catalytic processes exist for hydroisomerization, i.e., one based on the use of chlorinated Pt/Al₂O₃ catalyst and the other based on Pt/mordenite. The influence of acid leaching and steaming treatments on the catalytic activity and selectivity of Pt/mordenite was investigated, with emphasis on the increase in activity after acid leaching. The determination of the catalytic properties and different characterization measurements, i.e., N₂ physisorption, determination of acid site density, and transmission electron microscopy, of untreated crystalline mordenite and acid-leached mordenite was analyzed. Acid leaching significantly influenced the activity and selectivity of hydroisomerization of n-hexane over Pt/mordenite. The activity increased and the selectivity changed to favor primary products. Leaching selectively modified the mordenite structure, making more sites accessible for reaction and facilitating desorption of reaction products. The activity of untreated mordenite was limited by mass transfer effects, which vanished after modification of the zeolite structure by generation of a three-dimensional micropore structure and mesopores. The alleviation of intracrystalline diffusion limitation was the major factor in activity enhancement after acid leaching of Pt/mordenite.