Monolayers of 10,12-pentacosadiynoic acid (DA) were formed on aqueous subphases containing water-soluble polyamines such as polyallylamine (PAA), polyacrylamide and poly(N-vinvl acetoamide). DA was deprotonated in the Langmuir-Blodgett (LB) films transferred from the subphase containing PAA. However, DA remained protonated when the other two polymers were used. X-Ray photoelectron spectroscopy (XPS) revealed that PAA was incorporated in the DA LB films whereas the other two polymers were absent in the LB films. The ratio of DA molecule to the monomer unit of PAA was 1:2. Half of the monomer units of PAA were protonated. The atomic force microscopy (AFM) images of DA/PAA LB films showed a fingerprint-like pattern due to nanofibers with a width of 30 nm and a height of 7 nm. DA polymerized in the DA/PAA LB films on the irradiation of UV light. An absorption band at approximately 530 nm developed, followed by a small blue-shift of the band with the polymerization. Void regions appeared in the AFM images of the DA/PAA LB films accompanied by the polymerization. The band positions and the spectral profiles of CH. symmetric and antisymmetric stretching modes did not change significantly during the polymerization.