A model for the surface pressure isotherm and high frequency limit of dilational elasticity of a surfactant adsorption layers is proposed which assumes a two-dimensional compressibility of adsorbed molecules. While this compressibility has almost no significant effect on the isotherm, the elasticity is enormously affected. Experimental data for dodecyldimethylphosphine oxide (C12DMPO) show very good agreement with the model when a compressibility coefficient epsilon of about 0.007 m/mN is introduced.