Computational analysis of molecular dimensions and diffusion parameters of the various mono- and diisopropyl and tert-butylbiphenyl isomers within H-MOR and H-BEA zeolite framework has been performed using molecular mechanics and quantum mechanics methods. The calculated values have been compared with the experimental results obtained in the isopropylation and tent-butylation reactions of biphenyl with the corresponding alcohols over these large pore zeolites as catalysts. The calculated results are in good agreement with the experimental results achieved in the tent-butylation of biphenyl over H-MOR and H-BEA zeolites. The 4-MTBB and 4,4'-DTBB having a kinetic diameter of 5.8 Angstrom and the smallest energy barriers are formed preferentially over bout zeolites. In the case of the isopropylation reaction, the experimental results achieved over H-MOR are in a good agreement with the computational analysis, whereas over H-BEA, where the more hindered 3-MIPB and 3,4'-DIPB are preferentially formed despite of their higher kinetic diameters and energy barriers, the experimental results are somewhat different from the computational ones. The differences are related to the special architecture of BEA zeolite which allows the diffusion of relatively hindered molecules through its pores, contrary to the uni-dimensional structure of MOR zeolite. (C) 2002 Elsevier Science B.V. All rights reserved.