This paper reports the synthesis and the gas transport properties measured for polyarylates obtained by high temperature polycondensation of tetramethyl (TMBPA), hexafluoro (HFBPA) and tetramethyl hexafluoro (TMHFBPA) bisphenol A with a mixture of isophthalic (I) and terephthalic (T) acid chlorides (50 mol% initial I/T ratio). The polymers show high glass transition temperature (above 180 degrees C, good solubility in chlorinated solvents as well as excellent thermal stability (decomposition temperatures above 420 degrees C. The membranes produced from these I/T polyarylates are transparent and flexible films, and in addition, they have a gas permeability and selectivity balance of properties that falls on, for O-2/N-2, and close to, for CO2/CH4, the experimental "upper bound" limit pointed out by Robeson. The polyarylate membranes based on TMHFBPA I/T lead to the very attractive permeability coefficients of 182, 55, and 16 Barrers for the gases He, CO2 and O-2 with the corresponding still attractive ideal selectivity factors of 106, 32 and 5.7 for the pair of gases He/CH4, CO2/CH4 and O-2/N-2. The results reported in this paper are interpreted in terms of the fractional free volume and the most probable inter-segmental distance measured from density determinations and X-rays diffraction patterns. (c) 2007 Elsevier B.V. All rights reserved.