Model metalworking fluids, characterized by phase separation of functional components at elevated temperatures, were studied. Results of membrane filtration experiments with three oils of differing chemical nature confirm the feasibility of specific removal of finely dispersed contaminant oils without the simultaneous loss of active components from adequately formulated fluids. Prerequisites are that membrane materials and pore sizes be suitably chosen and that operating temperatures be kept below the cloud point of the least soluble component. The most suitable filter in the present study was a hydrophilic regenerated cellulose membrane with a NMWL of 100,000 dalton. Complete oil removal was attained if membrane pore sizes did not exceed 0.1 mu m It was also found that intrinsically water-soluble boundary lubricants of the polyglycol ether type are retained by membranes in the presence of PPG-1800, which serves as a precipitation promoter above the cloud point. This is of considerable practical importance since it offers the possibility of designing water-soluble boundary lubrication and extreme pressure additives which are activated by the presence of a inversely soluble component above its cloud point. The latter compound also acts as the principal hydrodynamic lubricant of the present model formulations. The usefulness of a new analytical tool for the rapid acquisition and imaging of data pertinent to changes in molecular aggregation and phase conditions was demonstrated.