Using a quartz crystal microbalance (QCM) measurement system, CO2 and CH4 capacity trapped by a solid-state ionic liquid, tettabutylphosphonium bis(trifluoromethanesulfonyl)imide [P-4 4 4 4] [NTf2], which was coated on the surface of a.. quartz crystal under pressures from 0.1 to 3 MPa at 30 degrees C, was calculated on the basis of the Sauerbrey equation. The frequency data under the pressure of 4 MPa are not applicable for CO2 measurements, because the ionic liquid film was destroyed and the frequency signal was perturbed possibly as a result of the reduced evenness of the [P-4 4 4 4] [NTf2] film after CO2 absorption. Besides, it is shown that both the gas sorption and CO2/CH4 selectivity, especially the mass-ratio-based selectivity, increase with pressure in the case of this solid-state ionic liquid film. During the ambient pressure, CO2/CH4 selectivity in [P-4 4 4 4] [NTf2] was lower than those in some reported fluid room-temperature ionic liquids.;The advantage of solid-state ionic liquids is an improved selectivity when the pressure increases, which is distinct from the results by fluid ionic liquids, for which the gas selectivity usually only slightly fluctuates with the pressure.