Membrane fouling mechanism in the vacuum membrane distillation (VMD) was studied both theoretically and experimentally by using ginseng crude extract aqueous solution as a model concentrate system. The concept of critical fouling operating conditions in VMD within which fouling is difficult to occur was introduced for the first time. By analyzing the forces acting on solute particles (microaggregates of biomacromolecules in the ginseng crude extracts solutions), three mathematical relations, namely the expressions of critical operating conditions, to distinguish between solutes depositing on membranes and those suspending in solutions, were derived. Then the effects of operating conditions including the cross-flow velocity, the feed temperature, and the vacuum degree on membrane flux were experimentally evaluated. The experimental results indicated that even under constant operating conditions, the membrane distillation flux could not maintain a stable level when one of following occurred: (1) the vacuum degree increased to its critical value, (2) the feed temperature increased to its critical value, or (3) the feed cross-flow velocity decreased to its critical value. The flux instability suggests that the critical flux had been exceeded and membrane fouling was taking place. These results confirmed the existence of critical fouling operating conditions in the VMD process. (c) 2010 Elsevier B.V. All rights reserved.