The desal 5 DL membrane (Dow Chemical, USA) was used to recover lactose from whey ultrafiltration permeate. Lactose and ions rejection and permeate flux were measured using ultrafiltered sweet whey as feed solution. Three different operation modes (total recycle mode, concentration mode and continuous diafiltration mode) were considered. Lactose retention was predicted by means of the Donnan Steric Partioning model (DSPM) and the Kedem-Spiegler model (KSM). The best fit for the total recycle and concentration modes was obtained with the KSM. Moreover, the KSM predictions were worse for the CDM. These models contribute to a better understanding of the transport mechanisms involved in nanofiltration processes and to optimize the process at industrial scale. The two models can be considered as a useful tool to improve the recovery, demineralization and concentration of lactose. (C) 2010 Elsevier Ltd. All rights reserved.