The membrane technique of nanofiltration (NF) was used to carry out partial demineralization of whey and milk ultrafiltration permeate (MUP). The experiments were carried out in a completely automated NF pilot plant. An aromatic polyamide spiral-wound membrane supplied by Osmonics (USA) was selected (model DK2540C). Feed, permeate and retentate were analyzed for lactose, protein, ashes, ions (Ca2+, Na+, K+, Mg2+, P, Cl-) and total dry extract content. Different operating conditions were considered. By means of NF total salt content was reduced at least by 30%, depending on the operating conditions and volume concentration ratio. Monovalent ions showed lower retention than divalent ones. Higher salt retention was obtained when whey was used as the feed stream. However, permeate fluxes were higher for MUP due to the lower amount of proteins, which can form a gel layer on the membrane surface. Protein and lactose permeation was observed to be negligible.