The use of reverse micelles coupled with ultrafiltration membranes for the separation of macromolecules dissolved in the cores of the reverse micelles using nearcritical and supercritical fluid solvents is described. This methodology allows one to address the separation of a wide range of polar molecules greatly extending the type of molecules that can be separated using only pure supercritical fluids. The solutes to be separated are initially dissolved in the reverse micellar solution and introduce into the pressure vessel containing the membrane. The surfactant and water core are passed through the membrane while the macromolecule selectivity is based on size and molecular weight. The ability for continuous recycle in an extraction system is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.