The oxygen and nitrogen permeability coefficients and oxygen/nitrogen selectivities of a series of isophorone-based polyarylates have been measured. The monomeric bisphenols used in this study are easily accessible in two steps from cheap, commercially available, isophorone (3,5,5-trimethyl-2-cyclohexen-1-one). The corresponding polyarylates can easily be prepared in high yield by means of the well-known interfacial polycondensation route. The effect of substituents (CH3 Cl, or Br) on all positions ortho to both hydroxyl groups in the bisphenol and the influence of the terephthalic acid/isophthalic acid molar ratio in the polyarylate on the oxygen/nitrogen separation performance have been studied in detail. It has been found that the polyarylate with four bromine substituents displays a significantly increased oxygen and nitrogen permeability coefficient, combined with a high oxygen/nitrogen selectivity, relative to conventional polyarylates. Obviously, the bulky 3,3,5-trimethylcyclohexylidene moiety in the bisphenol has structural features that hinder an efficient chain packing and reduce polymer segmental mobility. A further improvement in the gas permeability of these polyarylates can be achieved when the bulky t-butyl group is introduced on the 5 position in isophthalic acid. For example, the polyarylate prepared from the isophorone-based bisphenol with four methyl substituents and 5-t-butylisophthalic acid is characterized by an oxygen permeability coefficient of 34.7 Barrer!