tert-Butylation of diphenylamine (DPA) with tert-butanol (TBA) in the liquid phase was studied over various zeolite catalysts H-MOR, H-BEA, H-Y and H-ZSM-5 (H-MOR CBV 21A and 90A, H-BEA CP 811E, 814E and 814Q, H-Y CBV 720 and H-ZSM-5 CBV 5020) to evaluate whether zeolite catalysts can be efficient as an alternative to the other catalysts in preparation of tert-butylated diphenylamines. The influence of catalyst and the reaction conditions on tert-butylation of diphenylamine with TBA as alkylating agent on catalytic activity and para-selectivity was studied. Catalyst screening revealed that H-BEA and H-Y are the most suitable zeolite catalysts in the alkylation of DPA with TBA. Among them, H-BEA CBV 814E showed the best results in the catalytic activity (91% conversion of DPA), selectivity (99% selectivity to 4-TBDPA, 91% selectivity to 4,4'-DTBDPA) and dialkylation (0.62 4,4'-DTBDPA/4-TBDPA ratio). Subsequent optimization of reaction conditions in the tert-butylation of DPA with TBA over H-BEA CBV 814E showed that the most suitable reaction temperature is 180 degrees C. The study of influence of reactant's molar ratio showed that TBA/DPA molar ratio 4:1 is the most suitable, but the excess of TBA is so high that it produces significant amount of isobutylene dimer (15%). However, TBA/DPA molar ratio 2:1 is comparable to 4: 1, it is even better when considering the most important parameter-selectivity to 4,4'-DTBDPA and produces almost no isobutylene dimer (below 1%). The amount of catalyst charged into reaction system also significantly influences reaction parameters. Study shows that within wide range of catalyst charge (2.5% up to 30% as of DPA charge), the highest reasonable amount of catalyst with the best outcome can be obtained with 0.7 g (20% as of DPA charge) of catalyst charge in the reaction system. tert-Butylation of diphenylamine with tert-butanol over H-BEA zeolite catalyst appears to be an alternative to other catalysts in industrial preparation of tert-butylated diphenylamines. This paper presents the first original complex outlook on the tert-butylation of diphenylamine over zeolite catalysts. (C) 2007 Elsevier B.V. All rights reserved.