A procedure of analysis of protein adsorption isotherms at liquid interfaces is proposed in terms of the steady state pi-c curves and a thermodynamic model. Such practice is applied in the study of beta-casein, a random coil protein that seems to be very dependent on the nonpolar phase onto which it is adsorbed. First, differences in the interfacial behavior of the protein are drawn from the experimental data recorded at the air-water and tetradecane-water interfaces. Second, the application of the model suggests that the interface significantly affects the interfacial denaturation undertaken by the protein. More specifically, it provides the actual interfacial area occupied by the protein at both interfaces indicating a further unfolded state attained by beta-casein at the tetradecane-water interface. Finally, the procedure is also applied to literature data to clarify the apparent discrepancies found on the adsorption of beta-casein at different liquid interfaces.