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- A fabrication method of ultrathin polyimide network films was developed through polyion-complexation at the air-water interface and the template Langmuir-Blodgett (LB) technique. Two-dimensionally cross-linked LB films of poly(maleic acid-alt-methyl vinyl ether) (MA-MVE) and poly(allylamine) (PAA) were produced by using dihexadecyl phosphate (2C16P) as the monolayer template which was subsequently removed by thermal treatment followed by extraction. The polyion-complexed monolayers of three components, i.e., template 2C16P, co-spread PAA, and subphase MA-MVE, were formed at the air-water interface. Their monolayer properties were studied by the surface pressure-area isotherm. The monolayers could be transferred on solid substrates as Y type. The polyion-complexed LB films were characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The cross-linking procedure to form a polymer network was achieved by amide or imide formation through heat treatment under a vacuum. SEM observation of the film on a porous fluorocarbon membrane filter (pore diameter 0.1 μm) showed covering of the pores by four layers in the polyion complex state. Extraction by chloroform followed by heat treatment produced hole defects in the film.