Long chain fatty acids are usually undesirable contaminants in food oils and removal of these acids is an important task for the oil industry. In this paper the mechanism of transfer of fatty acids from liquid organic phase through flat porous membrane into water was investigated. The membrane pores initially were filled with the same organic solvent without fatty acid. It was demonstrated that the rates of transfer from octane into aqueous solution at pH 4 are inversely proportional to the distribution coefficients (K-d) and are mostly determined by diffusion in aqueous unstirred layers. Increase of pH of aqueous solutions from four to 12 results in a 20-fold increase of transfer rate for octanoic acid, and more than 1000-fold increase for oleic acid. Time lag Values necessary for the kinetics of fatty acid transfer to reach the steady state were very small for the transfer from octane, but they were much higher in the case of viscous mineral oil. These last values were in good agreement with Barrer's equation for diffusion through two unstirred layers. In both cases, there is no significant interface resistance for the transfer of relatively long chain fatty acids From organic solvent into water, Ion exchange processes take place in the aqueous phase and the thickness of the corresponding reaction layer decreases to zero at high pH. This final state is equivalent to the reaction, taking place at the interface, This method could be used for kinetic separation of short and long chain fatty acids at acidic pH and non-selective removal of different fatty acids into alkaline aqueous solutions. The membrane-based process does not need elevated temperature and probably is less energy consuming than distillation.