The concentration and time-dependence of interfacial pressure of nisin and β-casein at an n-hexadecane-water interface were evaluated by using DuNouy tensiometry. The two emulsifiers attained interfacial saturation at a bulk concentration of about 0.1 mg/ml, the reduction of the interfacial tension by nisin at that concentration being about equivalent to that of β-casein. The time dependence of interfacial tension recorded for each protein was described by using two kinetic models. In the first, the reduction of interfacial tension with time was considered to be a result of molecular penetration into the interface followed by rearrangement. Nisin exhibited more rapid penetration and rearrangement at the interface than did β-casein. In the second, the model allowed for the parallel, irreversible adsorption of protein into each of two states from solution, where state 2 molecules occupy a greater interfacial area and are more tightly bound than state 1 molecules. The extent of adsorption in state 1 and state 2 was determined to be highly concentration dependent for each protein ; adsorption occurs mostly in state 1 at high concentration and mostly in state 2 at low concentrations.