This paper reports some observations on the behaviour of a binary particle bed consisting of a mixture of small and large particles in the axial plane of a rotating drum. The effects of Froude number, particle diameter, filling ratio and initial particle concentration on axial segregation are evaluated using polymer powders. The results show that the number of segregation bands formed depends strongly on bed composition and processing conditions. Based on the experimental results, a segregation model to quantify axial dispersion is proposed. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.