The performance of a 300 kD UF membrane in separating highly concentrated and unstable O/W emulsions up to their phase inversion point was studied in this work. The membrane system was operated at constant fluxes ranging from 10 to 60 lmh. The flux vs. trans-membrane pressure relationship was used to determine the critical flux based on pressure. The oil concentration in the permeate as a function of flux was used to determine a critical flux based on oil penetration. The values of the critical fluxes, as determined by these two methods, were approximately equal. Separation efficiencies of 99.5% at fluxes below the critical flux were obtained. It was identified that the relationship between the critical flux for oil passage at 0.2 wt% oil vs. the concentration of oil in the emulsion was linear with an intercept corresponding to the phase reversal point. The filter cake at the surface of the membrane was found to be compressible with a pressure dependence ranging from 0.5 to 0.7. The oil breakthrough analysis indicated that the cake at the surface of the membrane had a noticeable yield stress similar to a Bingham fluid. The flux and oil permeation results were also explained based on the colloidal glass transition point of the oil droplets in the filter cake at the surface of the membrane. For highly concentrated, unstable, oil emulsions, operating at fluxes above the critical flux involves a penalty above that of increased pressure requirements that is reflected by a reduction in the separation efficiency of the membrane. (C) 2011 Elsevier B.V. All rights reserved.