New cardo poly(aryl ether ketone)s containing side phthalide groups and aryl ether ketones in different lengths have been synthesized and characterized in terms of their thermal, volumetric and gas transport properties to H-2, O-2, N-2, CH4 and CO2. The polymers show high glass transition temperature (218-420 degrees C), good solubility in chlorinated solvents and strong acids as well as excellent thermal stability (decomposition temperatures above 510 degrees C). The most permeable membrane studied shows permeability coefficients of 11 to O-2 and 72 to CO2, with ideal selectivity factors of 4.6 for the pair O-2/N-2 and 25 for CO2/CH4. The results, interpreted in terms of chain rigidity and chain packing ability, show that decreasing the length of the connector moieties between the cardo groups increases the fractional free volume, the glass transition temperature and the gas permeability coefficients. (c) 2006 Elsevier B.V. All rights reserved.