In a Langmuir monolayer of the surface active monomer octadecyl 3,5-diaminobenzoate (ODDB), stabilized at a surface pressure of 10 mN/m and a temperature of 23.7 degrees C at the air-water interface, oxidative coupling polymerization occurs when copper(II) chloride was added or already was present in the subphase. Even in the absence of the catalyst this reaction takes place. Deposition of the polymerized monolayer onto glass, zinc sulfide, or gold substrates, using the vertical dipping method, gave transfer ratios between 0 and 0.4 on the downstroke and 1 on the upstroke. The multilayer films on zinc sulfide and gold substrates were characterized by means of Fourier transform IR spectroscopy, and multilayer films on quartz were characterized by means of UV/vis spectroscopy. Collected monolayers were characterized by means of size exclusion chromatography and again Fourier transform IR and UV/vis spectroscopy. During the stabilization of the monolayer without addition of copper(II) chloride to the subphase, the area per molecule decreased very slowly. Addition of copper(II) chloride enhanced this effect. The reduction of the area per molecule is probably caused by the polymerization itself accompanied by a reorientation of the molecules in the monolayer at the air-water interface. After stabilization the area per molecule was found to be about 27 Angstrom(2) and electron micrographs showed the formation of a smooth monolayer. From this it has been concluded that a closely packed monolayer was formed. Fourier transform IR and UV/vis spectra of polymerized multilayer materials are analogous to the spectra of the azo polymer of ODDB prepared in bulk solution. Size exclusion chromatography revealed that the molecular weight and the molecular weight distribution of the coupling product with and without copper(II) chloride in the subphase were the same. The average molecular weight was somewhat lower than the high-molecular-weight part of the corresponding material prepared in bulk solution.