The concentration and separation of the amino acids N-benzyloxycarbonyl L-aspartic acid and L-phenylalanine methyl ester hydrochloride in organic solvents have been investigated using reverse osmosis membranes of two types of cellulose acetate, a nanofiltration membrane of polyamide-polyphenylene sulfone (PA-PPSO) composite and a gas separation membrane of polyimide composite in a stirred batch cell. The organic solvents used included primary, secondary, and tertiary alcohols, an ester, and a ketone. There were significant variations in permeate flux, solute rejection, and membrane stability. Usually the rejection of both amino acids was similar; however, certain membrane-solvent combinations gave significantly different levels of rejection. The highest rejection of amino acids (similar to 0.94) at the lowest pressure of 0.5 MPa was obtained with the PA-PPSO membrane using methanol as a solvent. The cellulose acetate membranes gave reasonable rejection and fluxes but the membrane stability was very poor. The performance of the polyimide composite membrane was good with ethanol but poor with other solvents. The PA-PPSO membrane with methanol as solvent appeared the most promising combination, and the separation performance according to concentration polarization was discussed.