A stochastic model is proposed for describing the size-density partition surface in dense-medium (DSM) gravity/centrifugal separators. The model is obtained by super-imposing a random walk on a simplified particle flow behavior inside the separator. The novel feature of the model is that it permits a description of the partition surface without incorporating the pivot phenomenon, i.e., without considering the partition number at the pivot point. The model also yields analytical expressions for the cut size, cut density and the ecart probable. The computed separation indices are in agreement with existing empirical relationships. Data from the published literature have been used to test the validity of the model in representing the partition surface. (C) 2003 Elsevier B.V. All rights reserved.