A periodic density functional theory study of the isomerization reactions of toluene and benzene catalyzed by acidic mordenite is reported. Reaction energy diagrams including transition-state energies of disproportionation and direct transalkylation reactions are presented and analyzed. Alternative reaction pathways have been considered. The use of periodic structure calculations allows analysis of steric constraints that occur within zeolite micropores. General rules concerning the influence of steric constraints in relation with reaction mechanism are described.