Recent experimental findings on the effect of a water soluble nonionic polymer in the subphase on phase transformations in an insoluble monolayer of pentadecanoic acid (PDA) are discussed. The experiments suggest that the influence of the polymer at large areas or low densities is primarily on the second virial coefficient of the surfactant monolayer. At high densities, competition between attraction of the monomers for the bare surface of the air-water interface and repulsion from closely packed PDA molecules results in displacement of the polymer from the interface.