Continuous enzymatic production of galactosyl-oligosaccharides (GOS) from a lactose substrate in an ultrafiltration membrane reactor (UMR) coupled with a nanofiltration separation system (CPNSS) was investigated. The overall rate of production over 4 h of continuous production was 80-104 mg GOS formed/U with an average residence time of 66 min, an initial lactose concentration of 300 g/L, an inlet pressure of 2.0 bar inlet pressure, and an outlet pressure of 1.5 bar. In the continuous diafiltration (CD) process, the concentration of various sugars and the relationship between the yield and purity of oligosaccharides was well predicted by mathematical models, the increased rate of sugar rejections was less than 10%, and the decreased rate of concentrations in the tank was less than 15%. In the CPNSS, 33.4 wt.% GOS was obtained in the UMR, and 1.24 kg of high-purity GOS (specifically, GOS purity of 57.2% and lactose content less than 20%) was achieved. This final yield was 80.1% GOS, which meets industry standards. (C) 2015 Elsevier B.V. All rights reserved.