This study deals with the estimation of rejection coefficient for small non-charged molecules in an aqueous solution, with reverse osmosis and ultrafiltration membranes in a stirred and an unstirred cell. In a stirred cell with high stirring velocity, the concentration near the membrane is close to the bulk concentration. Rejection is, therefore, known with good precision. On the other hand, in an unstirred cell, the dead volume under the membrane plays an important role and it is necessary to estimate, very precisely, the permeate solute concentration travelling through the membrane at any time. For each solute, the experiments carried out in a stirred and an unstirred cell permit to draw the same curve R = f(J(v)), where J(v) is the permeate flow velocity. These results seem to show that the dead volume correction used to calculate the solute concentration in the permeate, in an unstirred cell, is acceptable; they also permit one to hope that unstirred cell experiments can be used for the estimation of rejection coefficient when the estimation of the solute concentration near the membrane in a stirred cell is less precise (when polarization is high). (C) 2000 Elsevier Science B.V. All rights reserved.