Permeability measurements for oxygen and nitrogen were carried out on a series of structurally similar polyimides in order to define structure/permeability relationships. The polyimides were prepared from 5,5′-[2,2,2-trifluoro-1-(trifluoromethyl) ethylidene] bis-1,3-isobenzofuranedione (6FDA) and a variety of substituted methylene dianilines and benzidines. A correlation between the diamine monomer structure and polymer permeability was found, particularly with diamine monomer variations at the ortho positions of the diamine relative to the amino moiety. These correlations were semilogarithmic, relative to the summation of the substituent volumes. Direct correlations were observed between the measured properties of polymer density and interchain d-spacings and the oxygen permeability value of each of these polyimides. In addition, we observed that the calculated values of fractional free volume were related to oxygen permeability in a manner which enabled us to identity a polar component in the effects of these ortho substituents. © 1993 John Wiley & Sons, Inc.