Two samples of protonated mordenite with close nominal molar ratios (MRs) SiO2/Al2O3 equal to 15.0 and 15.2 are studied by various physicochemical methods. Both samples have the mordenite crystal structure but different morphologies and exhibit different dehydration patterns. According to the 1H, 27Al, 29Si NMR data, these samples show different local structural features, e.g. relative coordination of Si and Al containing tetrahedra. The complex analysis of 1H NMR spectra of hydrated and partially dehydrated samples and IR spectroscopy data shows that acid sites in both samples are mainly bridging OH groups and that the charge in H-MOR-15.0 prepared from the ammonia form is neutralized (in addition to the hydroxyl groups) by residual ammonium ions, while the charge in H-MOR-15.2 prepared by acid etching is additionally neutralized by mobile H+ and/or H3O+ cations.