We report the results of equilibrium molecular dynamics ( EMD) simulations of diffusion and sorption isotherms of CO2 and CH4 molecules in polyetherimide ( PEI). Amorphous PEI structures are generated by MD simulations and energy minimizations, and are characterized by their density, glass transition temperature, and the radial distribution function. The self- diffusivities of CH4 and CO2 in the polymer are estimated by EMD simulations. The simulations indicate oscillatory motion of the molecules inside the temporal cavities of the polymer and hopping from one cavity to another in the PEI matrix as the mechanism of gas transport in the polymer. The accessible free volume of the polymer is calculated with the use of a probe molecule. It exhibits strong temporal fluctuations which indicates the existence of correlations between these fluctuations and those in the excess chemical potential of the gases. The solubility coefficients of the gases, calculated by the test particle insertion method, and the single gas sorption isotherms, are within the reported range of the experimental values. The computed self- diffusivities D of the gases differ from the experimental data by about one order of magnitude, although the range of the experimental values for D is quite large. We also compute the sorption isotherms for binary gas mixtures of CO2 and CH4 in the polymer. (C) 2003 American Institute of Physics.