Abstract: 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. [Figure not available: see fulltext.]