The penetration dynamics of a dodecyldimethylphosphine oxid dissolved in the aqueous substrate into a Langmuir monolayer of eicosanyldimethylphosphine oxide and the equilibrium characteristics of the mixed monolayer formed are experimentally and theoretically studied. For the theoretical description of the low-density gaseous monolayers, the generalized Szyszkowski-Langmuir equation of state and the adsorption isotherm equation were applied. After the main phase transition, at which the gaseous phase starts to coexist with a condensed phase as a result of 2D aggregation, the monolayers are considered within the framework of a generalized Volmer equation and a quasichemical model for the a,aggregational equilibrium. The equations of state for the mixed monolayer are derived corresponding to different assumptions about the composition of 2D aggregates. The adsorption isotherm equations for soluble surfactant within the tightly packed monolayer are derived from the simultaneous solution of the corresponding equations of state and Pethica's equation. For the mixture of insoluble eicosanyldimethylphosphine oxide and soluble dodecyldimethylphosphine oxide, the experimental results were shown to agree well with the theoretical predictions based on the characteristics of these surfactants in the single-component solutions and monolayers. The model based on mixed aggregates is best suited to match the experimental values both for II-A and II-t isotherms.