The thermal and hydrothermal stabilities of EU-1 (EUO) framework zeolites were investigated as a function of their aluminium content in both the sodium and protonated forms. A course of decomposition patterns and other thermal effects were followed systematically using thermoanalytical (TG/DTA) methods. The results are discussed with the help of data from XRD, IR, XPS, TPD, 27Al MAS-NMR and equilibrium sorption studies. Samples subject to different calcination and steam treatments were examined. It was found that on increasing framework aluminium in EU-1 zeolites the thermal stability decreased and was less stable in its sodium form. The presence of water vapour during the thermal treatment tended to dealuminate the zeolites, changing their acidity pattern with a decrease in the surface Si/Al ratio. The catalytic activity and the product distribution for m-xylene isomerisation over steamed H/EU-1 catalysts were examined.