Mass transport algorithms using particle size distribution equations are common where molecular mass flux across a hydrodynamic layer is present. In this paper, we suggest a unique model and prove its effectiveness in modelling particle size distributions that vary dramatically in character and propose that the model may be useful for distributions of narrow and intermediate size ranges, more specifically, for ion exchange resin and fluidised bed catalyst, both of which are manufactured particulates. The model has a single fitting parameter and can be used in the frequency form for finite-element transport phenomena calculations. (c) 2006 Elsevier B.V. All rights reserved.