A set of five polyethers containing the highly fluorinated monomer, 1,3-bis(1,1,1,3,3,3-hexafluoro-2-pentafluorophenylmethoxy-2-propyl)benze ne (12F-FBE), were obtained by polycondensation with the monomers: biphenol, bisphenol F, bisphenol A, bisphenol AF, and a biphenol-substituted spirodilactam. Significant physical properties to characterize their application as membranes for gas separations processes were measured. They exhibit little crystalline domains (below 8%), very similar cohesive energy densities (CED) (with differences smaller than 1.8 (cal/cm(3))(1/2)), glass transition temperatures (T-g) above 94 degrees C, good thermal resistance (T-d > 350 degrees C), low specific volumes (V <= 0.6516 cm(3)/g), and fractional free volumes (FFV) above 0.170. Their permeabilities, at 35 degrees C and 2 atm, to He, H-2, O-2, N-2, CH4, and CO2 exceed those of commercial polysulfone (PSF) membranes. 12F-BISAF, the most permeable polyether with permeability coefficients (in Barrers) of 51 for He, 28 for H-2, 3.8 for O-2, and 12 for CO2, is also the material with the highest FFV. However, the observed relation between permeability and FFV confirms that besides FFV other features need to be taken into consideration to determine permeability-especially for the smaller permeants He and H-2. Concerning the ideal selectivity factors of these materials, they are in the same order of magnitude as those of PSF. The material identified as 12F-SDL is the most selective member of this series with selectivity factors to H-2/CH4, O-2/N-2, and CO2/CH4 of 84, 4.8, and 28, respectively. Such superior selectivity of 12F-SDL could be related mainly to inhibited segmental movements. These results are interpreted in terms of CED, FFV, and T-g together with their respective contribution of gas solubility and diffusivity to the overall selectivity coefficients.