Abstract: The performance of crosslinked copolymer of organosioxane (CGC) in forming homogeneous and composite membranes was studied. It is found that the CGC possesses outstanding membrane-forming properties and the surface texture of the homogeneous membrane is more fine. A pinhole free ultrathin membrane with thickness of 230-250 Å was prepared by spreading on water. The CGC, as a polymeric surfactant, can markedly improve the.mombraneformation of such polyolefins as poly-4-methylpentene-1, polybutadiene etc.With polydimethylsiloxane segment ranging from 60 to 70 Wt % the homogenteous membrane gave PO₂ value of 2.5-2.7×10^-8cm³ (STP)-cm/cm²·s·cm Hg and α_o2/N2 of 2. 2-2. 3. The composite membrane obtained by laminating CGC and modified poly-4-methylpentene-1 ultrathin membra nesalternately on Celgard 2400, showed permselectivity for oxygen/nitrogen, J_o2=3.43×10^-4cm³/cm²·s·cmHg, and α_o2/N2=2.52.