Kapton (R) 100HN polyimide membranes were heat treated at temperatures ranging front 623 to 1073 K. The precursor and the heat-treated membranes were characterized by thermogravimetric analysis, X-ray diffraction, Raman spectroscopy, density measurement nitrogen adsorption at 77 K, and single gas permeation. Compared with the precursor, the X-ray diffraction patterns did not change greatly when the membranes were heat treated at 623-748 K. The membranes treated at 623-723 K showed similar gas permeation rates as the original polyimide membrane whereas those treated at 773-873 K showed remarkable increases in the gas permeances due to the decomposition of the polyimide. Two minimum ideal selectivities were found for the membranes treated at 673 and 723 K. An increase in the temperature front 873 to 1073 K increased the crystallinity and decreased the interplanar spacing in the carbon Membranes which had microporous structures as shown by their Type 1 adsorption isotherms. The membranes treated at 773 K were deemed to be at an intermediate stage between polymer and amorphous carbon. By increasing the temperature from 773 to 873 K, the polyimide changed to a carbon membrane. The gas permeances for He, CO2, O-2, N-2, and CH4 were increased from 1.06 x 10(-9), 1.10 x 10 (9), 1.90 x 10 (10), 5.70 x 10 (11), and 5.55 x 10 (11) to 2.68 x 10(-8), 3.62 x 10 (8), 9.98 x 10 (9) 1.66 x 10 (9) and 5.17 x 10(-10) mol/(m(2) s Pa), respectively. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 235-242, 2009