While over silica alumina and non-shape selective zeolite catalysts such as FAU, m-xylene isomerizes at similar rates into o- and p-xylenes as expected from the classical intramolecular mechanism, over MCM-41 mesoporous aluminosilicates o-xylene is preferably formed (o/p greater than or equal to 2.5). A bimolecular pathway involving m-xylene disproportionation followed by transalkylation between trimethylbenzenes and m-xylene would be responsible for this preferential isomerization into o-xylene. In agreement with this proposal the addition of methylcyclohexane to the reactant causes an identical decrease in the rates of isomerization and disproportionation while over silica alumina the xylene isomerization which occurs through the classical intramolecular mechanism is not affected. The particular behaviour of MCM-41 aluminosilicates seems to be due to their unidirectional mesopore structure (shape selectivity effect) rather than their weak acidity.