The acid zeolite catalyzed ethylation and isopropylation of benzene with ethene and propene is investigated at the B3LYP/6-31G* level of calculation with a T-4-cluster representing the Bronsted acid site of the zeolite. After geometry optimization of reactants, transition structures, and products, a kinetic and a sensitivity analysis were used to model the reactions. These reactions proceed in one step, without formation of alkoxy species or charged intermediates. Both reaction rate constants and local HSAB properties indicate that isopropylation is favored over ethylation, in agreement with experimental observations.