Purification of oligosaccharides

A method for separating a tri- or higher oligosaccharide from a disaccharide which are dissolved in a feed solution comprising:

• (a) contacting the feed solution with a nanofiltration membrane with a molecular weight cut-off (MWCO) of 600-3500 Da ensuring the retention of the tri- or higher oligosaccharide and allowing at least a part of the disaccharide to pass, wherein the membrane is a piperazine-based polyamide membrane and comprises an active layer of polyamide, a MgSO4 rejection of 50-90%, and a NaCl rejection not more than 30%;
• and
• (b) collecting a retentate enriched in the tri- or higher oligosaccharide.