A concept is presented to determine the osmotic pressure of amino acid solutions by altering the stoichiometric coefficient of the van't Hoff law corresponding to the dissociation state of the amino acids. The osmotic pressure is greatly affected by the feed pH, which significance has been overseen in membrane permeation studies. Nanofiltration experiments were carried out in order to investigate the effect of osmotic pressure on the permeation of amino acids through polymeric membrane. L-Alanine aqueous solutions of various feed pH (6.0, 9.7 and 11.1) were concentrated in discontinuous operation mode with complete retentate recycle. It has been stated by many authors, that polymeric nanofiltration membranes possess higher rejection for amino acids in negatively charged form than for zwitterions. In our study we demonstrate that this tendency can be reversed in higher concentration range. (C) 2007 Elsevier B.V. All rights reserved.