The UV spectra of mixed monolayers were measured to gain insight into the interaction between amphiphiles with heterocycles, The 1:1 mixture of 5-(4-dodecyloxyphenyl)pyrimidine-2-carboxylic acid (1) with one of three amphiphiles, 2-(4-dodecyl-oxyphenyl)pyrimidine-5-carboxylic acid (2), 4'-dodecyloxybiphenyl-4-carboxylic acid (3). stearic acid (4) was spread on pure water, and the pi-A isotherms and UV spectra were measured. It became clear that in the 1:1 mixed monolayer of pyrimidine compounds 1 and 2. each component molecule was mixed at the molecular level, whereas in mixed monolayers of both 1 with 3 and 1 with 4, phase separation occurred. We considered that the difference in the mixing state of 1 and 2 from that of 1 and 3, or 1 and 4 can be ascribed to the stabilization by electrostatic interaction between pyrimidine rings.