In this study, the adsorption of methane (CH4) capacities onto natural mordenite obtained from Izmir, Turkey, and its cationic forms (CuM, AgM, FeM, and HM samples) were investigated at the temperatures of 0 and 25 degrees C up to 100 kPa. Natural and modified samples were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared (FT-IR), thermogravimetry (TG-DTG), differential thermal analysis (DTA), scanning electron microscopy (SEM) coupled with energy dispersion spectroscopy (EDS), and N-2 adsorption methods. Quantitative XRD analysis showed that the major component of the natural zeolite was mordenite, together with minor amounts of quartz, feldspar, and clay mineral. The specific surface area and microporosity of the mordenite sample decreased notably after Ag cation exchange treatment. It was found that the adsorption capacity and the affinity of CH4 with mordenite samples depended mainly on the type of exchanged cations and increased as HM < FeM < CuM < M < AgM for 25 degrees C. The uptake of methane increased as HM < FeM < CuM < AgM < M for 0 degrees C. Capacity of mordenites for CH4 ranged from 0.237 mmol g(-1) to 0.528 mmol g(-1).