Abstract: The invention provides a process for fractionation of compounds which in essentially non-polar solvents form mixed or co-micelles and hence cannot normally be fractionated by membrane filtration. By dissolving a mixture of such compounds in a solvent system in which selective micelles formation occurs, and contacting the solution under pressure with a semipermeable membrane to separate the mixture into a permeate and a retentate, fractionation of the compound mixture can be achieved, i.e. a fraction enriched in one of said compounds can be recovered from the permeate and/or the retentate by removing solvent therefrom. The new process is in particular useful for fractionating phosphatides-containing mixtures into a fraction enriched in phosphatidyl choline by ultrafiltration of a solution of the mixture in a solvent system comprising an essentially non-polar solvent and a polar solvent, e.g. hexane-ethanol, etc.