Surface or disjoining forces between protein layers adsorbed at the air/water interface were measured in single, isolated films. Two proteins, beta-casein and bovine serum albumin (BSA), were investigated under varying conditions of pH, ionic strength, and degree of aging at the interface. Force-distance curves were determined with a modified thin-film balance and interferometer, using both equilibrium and dynamic methods. Dynamic surface tension and ellipsometry data for beta-casein and BSA adsorption at the air/water interface are also reported. Charged beta-casein and BSA molecules do not strongly adsorb at the air/water interface. Accordingly, stable films were observed when electrostatic interactions were screened or the proteins were near their isoelectric points. For beta-casein, the force-distance curves indicated a transition from an outer to an inner branch at a distance equivalent to the diameter of beta-casein. This depended on the interfacial structuring of adsorbed beta-casein multilayers at the air/water interface. Upon thinning of the film, BSA retained its native dimensions, whereas beta-casein did not. As a consequence, fresh beta-casein films are more stable against rupturing than are BSA films. Aging imparted mechanical rigidity to the interfaces, causing nonuniform film drainage and nonequilibrium, trapped dimples. For films of intermediate degree of aging, black-film formation was observed through the formation of noncircular thin spots. Prolonged aging resulted in the development of interfacial aggregated networks and in films of variable thickness that did not respond to changes in capillary pressure.