A new polymeric precursor, poly(phthalazinone ether sulfone) (PPES), was used as precursor to prepare carbon membranes via stabilization and pyrolysis. The thermal stability of PPES was determined by thermogravimetry. The evolution of functional groups of membranes was monitored by ATR-FTIR during the formation process of carbon membranes. The carbon structure of the resultant carbon membranes was analyzed by X-ray diffraction technology. The gas permeation property of carbon membranes was tested by pure gases, H-2, CO2, O-2, and N-2. The results show that PPES is a highly thermally stable polymer with the char yield of 38.2 wt % at 700 degrees C in nitrogen. The functional groups of PPES gradually disappeared by forming a graphite-like structure in the membrane matrix during pyrolysis. At the test condition of 0.1 MPa and 30 degrees C, the gas permeabilities of H-2, CO2, O-2, and N-2 for carbon membranes prepared at the stabilization and pyrolytic temperature of 440 and 650 degrees C are 610.13, 439.90, 146.98, and 26.95 barter (1 barrer = 10(-10) cm(3) (STP) cm(cm(2) s cmHg) = 3.35 x 10(-16) mol m/(m(2) s Pa)), together with the selectivities of gas pairs H-2/N-2, CO2/N-2, and O-2/N-2 of 22.6, 16.3, and 5.5, respectively.