Carbon molecular sieve (CMS) membranes containing alkali metal ions (Li + , Na + , and K + ) were prepared by the pyrolysis of metal-substituted sulfonated polyimide (M-SPI) precursors. We have determined the effect of substituted metal ions in the polymeric precursor on the gas separation performance of CMS membranes containing metal ions. Thermogravimetric analysis showed that thermal stability of sulfonated polyimide was improved by substituting metal ions in the polymeric precursor. In single gas permeation experiments using He, O 2 , and N 2 gases having kinetic diameters from 2.6 to 3.64 Γ
, the gas permeabilities through the M-SPI membranes increased in the order: Li-SPI < Na-SPI < K-SPI. The presence of metal ions in their carbonized membranes also affected their gas permeation properties. The interplanar d spacing of the CMS membranes increased with increasing ionic radius of the substituted metal ion, and this was the cause of the effect on the gas permeation properties. At a constant concentration of metal ions, the gas permeabilities increased with ionic radius of substituted metal ions (Li + < Na + < K + ), whereas the ideal gas separation factor decreased in the same order. A CMS membrane pyrolyzed at 590 β’ C derived from K-SPI precursor showed maximum gas permeabilities for He, O 2 , and N 2 of 248, 6.8 and 1.7 Barrer, respectively, while one pyrolyzed at 590 β’ C derived from Li-SPI showed maximum He/N 2 and O 2 /N 2 mixture selectivities of 174 and 5.7.