tert-Butylation of biphenyl with tert-butanol has been studied over large-pore H-Y (15) and H-beta (25) zeolites under liquid phase conditions using cyclohexane as a solvent. The identification of main and by-products has been performed by GC-MS analysis. The main products of the alkylation reaction were mono-tert-butylbiphenyls and di- (tert-butyl)biphenyls with preferential formation of para- and para,para'-isomers. Higher allcylbiphenyls with alkyl group from C-4 to C'(10) were identified as by-products, These compounds can be formed by alkylation of biphenyl and/or mono-tert-butylbiphenyls with different alkenes, generated by dehydration of tert-butanol, followed by oligomerisation and disproportionation of the formed isobutylene. A peculiar effect of the small change in dimensions of micropores of H-beta (elliptic pores 0.76 nm x 0.64 nm) in comparison with H-Y zeolite (circular pores 0.74 nm) has been reflected by the transalkylation of mono-tert-butylbiphenyls to cyclohexylbiphenyls and methylcyclopentylbiphenyls over the more acid H-beta zeolite catalyst, whereas this transalkylation has been not monitored in the experiments with H-Y zeolite. The occurrence of transalkylation reactions has been supported by thermodynamic analysis and calculations.