Monolayers of synthesized, polymerizable liquid crystals which have a hydrophilic cyanobiphenyl group with various alkyl chains spread at the air-water interface were investigated. The liquid crystals mixed with a poly(amic acid) alkylamine salt were spread, and compressed with two barriers to obtain surface pressure-area (pi -A) isotherms. The miscibility of the mixed monolayers was studied by calculating the change in the Gibbs free energy of the mixed monolayers. The mixed monolayers were exposed to a UV lamp at a constant surface pressure. During both processes of compression and UV irradiation, the monolayers were monitored by a Brewster angle microscope (BAM). The in situ polymerization was monitored by a multi-channel photo detector and the extent of the reaction was calculated. The change in the area of the mixed films showed that UV irradiation induced both the in situ photopolymerization causing phase separation and degradation of the monolayer of poly(amic acid) alkylamine salt at the air-water interface.