This work presents a study of concentration of apple and pear juice by nanofiltration at low pressures. The aim was to find a proper membrane and the most adequate process conditions to reach the highest concentration degree of the juices, pressures between 8 and 12 bar and temperatures of 25-35 degreesC were considered. To understand better and to optimize the process, nanofiltration membranes were characterized by using atomic force microscopy (AFM) and scanning electron microscopy (SEM); also an evaluation of irreversible fouling was carried out. The results indicated that in membrane selection both retention and permeation values should be considered, and that irreversible fouling of fruit juices is relatively low (30 +/- 8%). AFM and SEM analysis allowed visualization of membrane topology (mean pore distribution, roughness and distribution of separation-layer particles) and morphology (thickness of membrane layers). We concluded, that roughness does not depend on structure density and that less rough membranes had lower yield of irreversible fouling. Finally, influence of fructose on fouling performance was checked. (C) 2003 Elsevier Ltd. All rights reserved.