Tert-butyl and di-tert-butyl were added as pendant groups to the ether-ether phenyl ring of poly( ether ether ketone), PEEK. tert-butyl PEEK, TBPEEK, was amorphous and di-tert-butyl PEEK, DBPEEK, was semicrystalline. However, a 2 : I random copolymer of TBPEEK and DBPEEK, TBDBPEEK, was amorphous. Gas transport of N-2, O-2, CH4, and CO2 through amorphous films of PEEK, TBPEEK, TBDBPEEK, and tetramethylbiphenyl PEEK were determined at 35 degrees C and at pressures to about 15 atm. The results support previous observations that tert-butyl and tetramethylbiphenyl groups are very effective in disrupting chain packing in the polymer. For the present polymers, these substitutions led to a 5-18-fold increase in permeability, and, in some cases, at no loss in permselectivity.