The transformation of pure xylenes, of pure trimethylbenzenes (TMB), and of mixtures of m-xylene (greater than or equal to 95 mol%) and 1,2,4-trimethylbenzene was carried out at 623 K on a HY zeolite with a framework Si/Al ratio equal to 16. TMB isomerization was 4 to 10 times faster than xylene isomerization. The disproportionation of 1,2,4-TMB was 4 to 7 times faster than the disproportionation of p-xylene and of the other TMB isomers and 30 and 55 times faster than that of m-xylene and of o-xylene. The addition of 1,2,4-TMB to the m-xylene reactant caused a significant increase in the formation of xylene isomers and particularly of o-xylene owing to a rapid transalkylation between 1,2,4-TMB and m-xylene. The high rate of the latter reaction rendered possible, besides the unimolecular isomerization process, a direct isomerization of m-xylene through two successive bimolecular reactions, m-xylene disproportionation followed by transalkylation between the trimethylbenzene produced and m-xylene. From a simple model it was shown that the para/ortho selectivity of the bimolecular m-xylene isomerization was very different from (much lower than) that of the unimolecular isomerization. This explains why in a series of HY zeolites with different framework Si/Al ratios the greater the disproportionation to isomerization rate ratio (DIT) the lower the para/ortho selectivity. The value of the para/ortho selectivity of the unimolecular process was determined accurately from the extrapolation at D/I = 0 of the para/ortho ratio. Low values of D/I were obtained by inhibiting selectively m-xylene disproportionation by adding methylcyclohexane to the reactant. It is shown that the relative proportion of unimolecular and bimolecular mechanisms can be estimated from the experimental values of the para/ortho ratio.