The diffusive behavior of pure and mixed (CO2)-C-13, methane, and ethylene in poly(4,4'-hexafluoroisopropylidene diphthalic anhydride-2,3,5,6-tetramethy1-1,4-phenylenediamine) membranes is investigated using a pulsed gradient spin echo NMR method. Also, proton and C-13 spin-lattice and spin-spin relaxation times are measured. In all cases, the relaxation times of sorbed gases are well described assuming a single component, indicating that gas molecules in the membrane recognize one environment. If there is more than one, they are in fast exchange between them, and average values of relaxation times are observed. The diffusion coefficient of (CO2)-C-13 is independent of the diffusion time, exhibiting a Fickian behavior in all cases. Meanwhile, those of methane and ethylene vary with the diffusion time, and data analysis was performed considering their non-Fickian behavior. The diffusion coefficient of methane increases in the presence of (CO2)-C-13 and ethylene, suggesting that the sieving properties of the glassy polyimide membranes are compromised even at low concentrations of condensable gases.