Oil-in-water emulsions stabilized by caseins were made using a microfluidizer. The average diameters of the emulsion droplets were determined by quasi-elastic light scattering. A technique of enzymatic breakdown of the adsorbed protein layer together with light scattering was employed to study the thickness of the adsorbed layers. It was found that it was possible to make emulsions with a range of concentrations of protein on their surfaces, and that there was a minimum surface coverage (about (1 \mathrm{mg}) (\mathbf{m}^{-2}), which was required to produce stable emulsions, as well as a maximum coverage (about (3 \mathrm{mg} \mathrm{m}^{-2}) ), where no further protein could be accommodated. These two limiting values of the adsorbed amount corresponded to two values of the hydrodynamic thickness of the adsorbed layers. For the lowest adsorption, the layers were about (5 \mathrm{~nm}) thick, which probably represents the maximum spreading of the protein molecules on the surface. When adsorption is maximal, the adsorbed layer was about (10 \mathrm{~nm}) thick so that the casein molecules protrude further into the solution. The results demonstrate that it is possible for flexible proteins like caseins to adopt a number of different conformational states when it is adsorbed to an oil-water interface. (0) 1993 Academic Press, Inc.