Catalytic activity of various molecular sieves, activated bentonite and BF3-etherate was investigated in the condensation of aniline to diphenylamine. Among the tested catalysts, the zeolite Beta with a Si/Al atomic ratio of 12.5 exhibited the highest activity (ca. 34% aniline conversion) and selectivity (above 98%) for diphenylamine formation. Zeolites Beta with higher Si/Al atomic ratio were less active but the selectivity to diphenylamine remained almost the same. It is assumed that the differences in the catalytic activities of molecular sieves are not caused only by the differences in the pore geometry but depends also on the concentration of acid sites. The type of acid sites, Bronsted or Lewis sites, does not significantly influence the catalytic performance. Strong acid sites prepared by activation of bentonite with hydrochloric acid are probably responsible for the formation of by-products and the resulting catalyst is, thus, less selective to diphenylamine formation. (C) 2003 Elsevier B.V All rights reserved.