Dealumination of ferrierite type zeolite was studied using aqueous solutions of hydrochloric acid of different concentrations. The course of the reaction was monitored by XRD, sorption, NMR, XPS and TPD of ammonia. The catalytic properties were studied using m-xylene transformations as a test reaction. A mild treatment with the diluted acid exerts a significant effect on the reaction path. The selectivity towards p-xylene formation on the dealuminated samples was strongly preferred and much greater than thermodynamic equilibrium data. Simultaneously, disproportionation of m-xylene yielding toluene and trimethylbenzenes was suppressed. (c) 2005 Elsevier B.V. All rights reserved.