A theoretical model is developed which describes the penetration of amphiphiles (surfactants, polymers, proteins) dissolved in an aqueous substrate into a Langmuir monolayer. Penetrated monolayers capable of 2D aggregation are treated theoretically within the framework of the generalized Volmer equation of state and the quasichemical model of the aggregational equilibrium. The equations of state are derived far mixed monolayers, just as the soluble amphiphile adsorption isotherm equations by the simultaneous solution of the corresponding equation of state and Pethica＇s equation. The theoretical model is used for the description of equilibrium protein penetration into Langmuir phospholipid monolayers where after the main phase transition point condensed phase structures are formed by 2D aggregation. In this case the general thermodynamic approach has to be extended to consider the existence of free charges and multiple states of the adsorbed protein molecules. The protein isotherm equation derived accounts for the 2D aggregation of the lipid. The experimental data for the lysozyme pentration into DPPC monlayers agree satisfactorily with the theoretical calculations. It is theoretically predicted that if the bulk concentration of lysozyme exceeds a certain limit; 2D aggregation of DPPC is induced.