A series of thianthrene-5,5,10,10-tetraoxide-containing polyimides with great chain stiffness were prepared and their gas permeation properties were investigated. TADATO/DSDA(1/1)-DDBT copolyimide, which was prepared from thianthrene-2,3,7,8-tetracarboxylic dianhydride-5,5,10,10-tetraoxide (TADATO), 3,3',4,4'-diphenylsulfonyltetracarboxylic dianhydride (DSDA) and 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT), displayed much higher CO2 permeability coefficient (P-CO2) than both DSDA-DDBT and TADATO-DDBT homopolyimides, but their ideal selectivity (P-CO2/P-N2) were similar. The higher CO2 permeability coefficient of TADATO/DSDA(1/1) -DDBT was attributed to the higher diffusion coefficient as well as a little higher solubility coefficient. The separation performance of TADATO/DSDA(1/1)-DDBT toward CO2/N-2 fell above the "upper bound" line, indicating the highest separation performance among the glassy polymer membranes developed so far. Furthermore, this polyimide showed a significantly higher selectivity of CO2 over N-2 for mixed gas permeation than for single gas permeation (single gas: P-CO2/P-N2 = 35, mixed gases: P-CO2/P-N2 = 49, at 35 degreesC and 2 atm). The "dual-mode" model was used to describe gas sorption and transport behaviors in the membrane.