A new type of aromatic poly(ether ether ketones) (4a-4c) were prepared by polycondensation of different amounts of 4,4-bis(4-hydroxyphenyl)pentanoic acid (1) and 2,2’-isopropylidenediphenol (2), respectively, with 4,4’-difluorobenzophenone (3). The homocondensate (4a) from 1 and 3 shows a high T-g (190 degrees C) and a loss in weight of 6.8% at 350 degrees C. The amorphous polycondensates 4a-4c were cast from NMP or DMAc solutions to produce homogeneous, self-standing films that were clear and colorless. In the case of the membrane from 4a a preferred permeability of O-2 from a O-2/N-2 gas mixture was found (alpha = 2). Additionally, the carboxylic group has been condensed with several amines via acid chlorides. A strong influence of the side chain length on the glass transition temperature was detected.