The gas transport properties of 6FDA copolyimide membranes have been studied by examining their interaction such as the charge transfer complex between donor and acceptor molecules or the pi-pi aromatic stacking and their molecular motion in the solid-state. The interaction and the molecular motion in the membranes have been measured using fluorescence spectroscopy and solid-state C-13 NMR spectroscopy, respectively. The gas permeability and selectivity of CO2, O-2, N-2, and CH4 for the membranes have been measured at 35 degrees C and at pressures up to 10 atm. The gas permeability of the copolyimide membranes was significantly dependent on the gas diffusion, and the diffusion selectivity was a principal factor that dominates the determination of the gas selectivity in the membranes. A good correlation was found between the gas transport properties and the carbon spin-lattice relaxation time associated with the molecular motion of the CF3 group in the copolyimide membranes.