Permeabilities to CO2 in an equimolar mixture of CO2 and air for a homogeneous poly(4-methyl-1-pentene) membrane were measured at various upstream gas pressures. The permeability to CO2 for the mixed gas was smaller than that for pure CO2 at the same CO2 upstream pressure. The permeability to O-2 for the mixed gas increased with increasing upstream gas pressure, although for pure O-2 the permeability was independent of pressure. Such experimental evidence is consistent with the free-volume model prediction. The increase in permeabilities for the coexisting O-2 might result from the membrane plasticization action of sorbed CO2. The depression of CO2 permeability in the gas mixture case might be ascribed to the hydrostatic pressure effect applied to the system, as suggested by the free-volume model.