New immunoprotecting membranes were prepared by spin coating the amphiphilic random multiblock copolymers of poly(ethylene glycol) (PEG) and poly(tetramethylene ether glycol) (PTMEG) or poly(dimethyl siloxane) (PDMS) on porous Durapore® membrane. The copolymer coating was intended to make a biocompatible, immunoprotecting diffusional barrier and the supporting porous substrate was for mechanical stability and processability. By filling Durapore® membrane pores with water, the penetration of coating solution into the pores was minimized during the spin coating process. A single coating process produced a completely covered thin surface layer (∼1 μm in thickness) on the porous substrate membrane. The permselectivity of the coated layer was influenced by PEG block length, polymer composition, and thickness of the coating layer. A composite membrane with the coating layer prepared with PEG 2 K/PTMEG 2 K block copolymer showed that its molecular weight cut-off at day 40 based on dextran was close to the molecular size of IgG (M w = 150 kDa). However, IgG permeation was detected from protein permeation test, while glucose oxidase (M w = 186 kDa) was not permeable through the coated membrane.