A process for isomerising xylene using a new family of related crystalline aluminosilicate zeolites has been developed. These zeolites are represented by the empirical formula: M.sub.m.sup.n+ R.sub.r.sup.p+ Al.sub.(1-x) E.sub.x Si.sub.y O.sub.z where M is an alkali or alkaline earth metal such as lithium and strontium, R is a nitrogen containing organic cation such as tetramethyl-ammonium and E is a framework element such as gallium.