Partially Li+ exchanged Na-ZSM-25 zeolites (LNZ25) were incorporated into the commercial polymer Matrimid (R) 5218 to yield a mixed-matrix membrane (MMM) via solvent-casting. ZSM-25 has exclusive admission of CO2 and complete exclusion of CH4 and is therefore an ideal candidate for MMMs. The fillers were evenly dispersed in the polymer matrix with good adhesion. The separation performance of the fabricated MMMs with 0-15 wt% filler loadings are determined by both single and binary gas permeability using a constant pressure variable volume apparatus. Single gas permeation tests on the MMM with 5 wt% filler loading show exceptionally high CO2/CH4 selectivity (169 at 5 bar) and a plasticization-resistance-like behaviour, i.e. the selectivity increases as the feed pressure increases. Binary gas measurements indicate the CO2/CH4 selectivity of the MMM is constant with an increase of pressure while the selectivity of the pristine Matrimid decreases with increased pressure. The exclusive admission of CO2 and the rejection of CH4 of the LNZ25 contributes to enhanced permeability and plasticization-resistance-like performance of the MMM.