Fully cyclized polyimides based on octadecyl- or hexadecyl 3,5-diaminobenzoate and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride or 4,4'-oxydiphthalic anhydride were synthesized and deposited by the Langmuir-Schaefer technique onto a poly(1-trimethylsilylprop-1-yne) support. Composite membranes thus prepared show selective gas permeation for N-2, O-2, CO2, H-2 with permeation rates in the range of 10(-5) to 10(-7) cm(3) (STP)/(cm(2).s.cmHg).