Sorption equilibria and permeation rates for CO2, O2 and N2 in poly(ether imide) (PEI) membrane which has a glass-transition temperature of 216-degrees-C and in poly(ether ether ketone) (PEEK) (a typical crystalline polymer, crystallinity=0.4) membrane, whose glass-transition temperature is 143-degrees-C, were measured at 25, 30 and 40-degrees-C and at pressures up to 3 MPa. The pressure dependence of the mean permeability coefficients of PEEK membrane to 02 and CO2 obeyed the conventional dual-mode mobility model, whereas that of PEI membrane to CO2 did not follow the conventional dual-mode mobility model. A modified dual-mode mobility model which takes into account four kinds of diffusive movements was satisfactorily used to simulate the pressure dependence.