A homogeneous dense membrane of bisphenol A poly(ether ether ketone ketone) (PEEKK) has been prepared by a solution casting technique with chloroform as solvent and characterized by wide-angle X-ray diffraction and tensile tester. Permeating characteristics of five pure gases through the membrane has been described systematically. The permeability, diffusivity, solubility, and their selectivities of oxygen, nitrogen, carbon dioxide, methane, and hydrogen through the membrane have been measured by a change in operating temperature and upstream pressure in a time-lag apparatus. A continuously enhanced permselectivity for gas pairs of carbon dioxide/methane, and hydrogen/nitrogen. an enhanced diffusivity selectivity for gas pairs of oxygen/nitrogen, carbon dioxide/methane, as well as a decreased permeability and diffusivity for five pure gases, have been observed with decreasing operating temperature. The solubility of five gases in the PEEKK membrane increases with decreasing temperature. The solubility selectivity hardly ever varies with operating temperature. Especially, the permeability, diffusivity, solubility, and their selectivities in the PEEKK membrane almost maintain constant with changing the upstream pressure. The highest oxygen/nitrogen, carbon dioxide/methane, and hydrogen/nitrogen selectivity coefficients were, respectively, equal to 7.06, 23.1, and 106.1. A relationship between the permeability or diffusivity and molecule diameter of the gases across the PEEKK membrane was discussed. The gas solubility in the membrane is believed to be a linear function of the critical temperature of the gases.