Compression isotherms of mixed monomolecular layers at the water-air interface, formed of lecithin (DPPC) and an organometallic compound (Me₃PbCl , Me₃SnCl, Et₃PbCl, Et₃SnCl) at various molar fractions (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were studied. It was found that compression isotherms of the pure organometallic compounds (x = 1.0), in the range of mean molecular areas of 1.0 nm² and approximately 0.15 nm², did not show any practical increase in surface pressure. The isobaric relationship between mean molecular areas and molar fraction of an organometallic modifier are linear for most of the pressures chosen, which follows from the analysis of the experimental isotherms obtained and demonstrates the ideal behavior of the system under study. All the compounds studied form nearly ideally-behaving systems with lecithin. Presumably, the effects observed may be the result of molecular aggregation on the water phase surface, which in a mixed monolayer may lead to separation of the phases: organometallic compound - lecithin.