We present results of an air/water interfacial study of the calcium-binding protein, calbindin D-9k. The isotherm displays two inflection points, One at low surface pressures earmarks the limit of the gaseous state of the monolayer and represents the coexistence of two phases, The second inflection point appears at higher surface pressures and is likely associated with the expulsion of segments from the interface, Results found with a film formed with a mutant protein (three charged amino acids are replaced by neutral groups) suggest that the monolayers are not greatly expanded at the pH of the experiments, However, with the mutant protein monolayer both inflection points are substantially diminished. In this paper we also demonstrate the utility of plotting the area derivative of the surface pressure in the interpretation of monolayer behavior.