A carrier-mediated countertransport process for the extraction of amino acids is reported. The permeation of L-Phe through a liquid membrane, constituted by a solution of ALIQUAT-336 in toluene and supported by a hydrophobic microporous membrane, was studied. The best operating conditions were deducted by considering the effect of hydrodynamic factors, carrier concentration in the membrane phase, etc. The effect of impurities such as phosphates and tyrosine on the flux of Phe and selectivity of the carrier was evaluated.