This study investigates fouling of membranes during membrane distillation (MD) of two model dairy feeds - skim milk and whey, as well as their major single components. Every MD experiment was conducted for 20 h at 54 degrees C feed inlet temperature and 5 degrees C permeate inlet temperature using PTFE membranes. Performance was assessed in terms of throughput (flux) and retention efficiency. Skim milk flux was found to be lower but stable over time compared to whey. The study using single components as well as combinations thereof revealed that fouling was primarily driven by proteins and calcium, but only in combination. Lactose also played a role to a lesser extent in the protein/membrane interactions, possibly due to preferential hydration, but did not interact with the membrane polymer directly. However lactose was found to deposit once an anchor point to the membrane was established by other components. Skim milk showed strong adhesion from its principle proteins, caseins; however salts were needed to form a thick and dense cake layer. Caseins seem to form a layer on the membrane surface that prevents other components from interacting with the membrane polymer. Whey proteins, on the other hand, deposited to a lesser extent. In general, membrane distillation was found to be a process that generates high quality water with retention of all tested components > 99% while simultaneously concentrating whey or skim milk. (C) 2013 Elsevier B.V. All rights reserved.