The results of this work show that when Zn species are loaded in well-defined amounts onto the bifunctional catalyst Pt-HY and by using particular impregnation techniques, cracking is almost suppressed within the range of reaction temperature investigated. Our concept of trifunctional catalyst suggests that Zn2+ ions behave as sorption competitors of the zeolite acid sites so that the residence time of the alkylcarbocationic intermediates on these acid sites is shortened, reducing therefore the negative effect of cracking. This concept is equivalent to the "ideal" mechanism proposed by Guisnet et al., according to which, once the branched olefin is desorbed from the acid site, the probability to have the encounter with a new acid site is slight, in deep contrast with other "non-ideal mechanisms".