Thin emulsion films stabilized with p-casein were studied by microinterferometry. The data demonstrate that the film thickness and intersurface adhesion are influenced by the electrolyte concentration, the prehistory of the protein solution, pH and the presence of fatty acids in the oil phase. A major shift of the film parameters occurs in the presence of calcium ions. The decrease of the film thickness above 1 mM Ca2+ is consistent with earlier neutron reflectivity and hydrodynamic thickness measurements. The most interesting effect is the strong cross-binding of the film surfaces above a threshold concentration of 12 mM of calcium. Electrophoretic mobility data suggest that at this concentration one Ca2+ ion per two protein molecules is adsorbed on the outermost segments of the protein layers. The possible origin of the cross-binding is discussed on the basis of the structure of the adsorbed p-casein molecules. Data for the resolution of batch emulsions demonstrate that the effect of calcium observed with emulsion films is directly related to the stability of practical systems. Surface force apparatus measurements indicate that the effect, although attenuated, may also be present in suspensions of casein-stabilized solids.