The aim of this work was to study the concentration of licorice aqueous solutions using reverse osmosis (FT 30) and nanofiltration (BDX N-90) membranes. The effects of transmembrane pressure, feed temperature, feed pH and cross-flow velocity on permeate flux and rejection were determined. A lab scale cross-flow set-up using flat-sheet configuration membrane was employed for all experiments. SEM micrographs showed the changes in the cross-section of RO and NF membranes at various pH solutions and surface alteration fouled during operating time. The applied transmembrane pressure, feed temperature, feed pH and cross-flow velocity were varied from 6 to 14 bar, 25-45 degrees C, 3-11 and 0.5-3.2 m/s, respectively. The obtained rejection values varied between 93 and 99.6%. The optimum operating conditions for concentration of licorice aqueous solutions using FT 30 reverse osmosis membrane were 1.8 m/s cross-flow velocity, 12 bar transmembrane pressure, 35 degrees C of feed temperature and pH 5. For BDX N-90 nanofiltration membrane the optimum conditions were 2 m/s cross-flow velocity, 10 bar transmembrane pressure, feed temperature 35 degrees C and pH 5. The membranes were tested for 10 h. The permeate flux was gradually decreased during the operating time and eventually reached a constant value. (C) 2010 Elsevier B.V. All rights reserved.