Purification of fructo-oligosaccharides (FOS) by nanofiltration (NF) technology under different operation modes was studied theoretically and experimentally. The characteristic function of variable volume diafiltration (VVD) was proposed. For concentration, constant volume diafiltration (CVD) and VVD. the relationships between (i) FOS purity and dilution water usage and (ii) FOS purity and yield were obtained. The relationship between purity and yield was found to be independent of operation mode, when the membrane rejection properties were the same. While for a given purity, the dilution water consumption under VVD was found to be less than that under CVD. These findings were confirmed by the experimental results of FOS purification, performed with two spiral wound NF membranes. (C) 2004 Elsevier B.V. All rights reserved.