An empirical Weibull function is proposed to represent partition surface of gravity concentrators in terms of size-density attributes. Model derivation is keyed with observed pivot phenomenon. The model is found adequate to represent partition surface of gravity concentrators. The model yields analytical expressions for computing cut-density and ecart probable. The suitability of the model is examined using several sets of measured data from literature. (C) 2004 Elsevier Ltd. All rights reserved.