This study deals with a wiper for a molecular evaporator. Its wiping element has the shape of a cylinder with a screw thread with coarse pitch (lead) on its surface. The wiping element bears on the evaporating cylinder due to centrifugal force and rolls over it, bringing the liquid on the evaporating cylinder into an intensive complex movement. The screw thread on the wiping element surface increases the permeability of the liquid through the element, while restricting the formation of a longitudinal wave which develops in front of the element. An analysis of residence time distribution curves shows that there are three different liquid how regimes on the evaporating cylinder. In this study, conditions of the formation of these regimes and their consequences are discussed. Optimum conditions for distillation can be reached at peripheral speeds of the wiper at which increased retention of a liquid on the evaporator occurs. This leads to an increased residence time. The increased residence time is a positive factor which makes efficient evaporation under gentle conditions at a lower evaporating temperature possible. (C) 2001 Elsevier Science B.V. All rights reserved.