Unit cell formulas (UCFs) of clinoptilolite, the main mineral of a ryolite tuff, were calculated from the chemical analysis, from cation exchange capacity using the method of Barrer et al. (1967), and from the molar cation exchange capacity. The last two methods provided similar results. Exact knowledge of exchangeable cations contents and valency of exchangeable Al-bearing species is needed for correct UCF calculation. A set of dealuminated samples was prepared from NH₄⁺ saturated tuff by its reaction with (NH₄)₂ SiF₆ for 1 to 24 hr at 95 °C in water suspension of ammonium acetate. Framework (FAl) and extra-framework (EFAl) aluminium were distinguished in the dealuminated samples due to their different dissolution rate in 4 M HCl at room temperature. Unit cell formulas of dealuminated samples calculated using the FAl instead of total Al content are believed to express more correctly the real composition of dealuminated clinoptilolite.