Competitive adsorption of proteins from several binary protein solutions to the air-water interface has been studied. With a few exceptions, the equilibrium composition of the saturated monolayer of mixed protein films at various bulk concentration ratios did not follow a Langmuir-type competitive adsorption model. The deviation from ideal behavior results from incompatibility of mixing of proteins in the film at the air-water interface. This immiscibility alters the ratio of the binding affinity of the proteins in a protein 1/protein 2/water ternary film compared to that in a protein 1/water and protein 2/water binary film. A method to determine the extent of incompatibility between two proteins in a mixed protein film has been developed. It is shown that the incompatibility index derived for 19 protein 1/water and protein 2/water systems studied show a linear relationship with the absolute difference between Flory-Huggins protein-solvent interaction parameters, that is, /chi(1s) - chi(2)/, of the constituent proteins. On the basis of the evidence, it is theorized that, because of incompatibility, proteins in a mixed protein film at interfaces may undergo two-dimensional phase separation.