The ammonium form of ferrierite (obtained from K,Na-ferrierite, Tosoh) was modified with aqueous solution of hydrochloric acid of different concentration. It has been demonstrated that ferrierite is amenable to HCl solution treatment, while retaining its high crystallinity (XRD, SEM). Dealumination proceeded smoothly and 60% of aluminium was easily removed from the sample just upon 0.25 M HC1 treatment. Si-29 NMR revealed that during the acid treatment the aluminium atoms were removed preferentially from Si(2Al) and Si(1Al) TB positions. Acidity of the samples was assessed by sorption of CO and ammonia and using FT IR spectroscopy. Studies of in-xylene transformations showed increased activity of the samples with increasing level of dealumination. Such behaviour was attributed to a particular morphology of the parent ferrierite. Dealumination of zeolite with the plate-like morphology yields a highly active material, coupled with high para-selectivity after the first acid treatment. (c) 2006 Elsevier B.V. All rights reserved.