The effect of polymer polydispersity on the fluid-fluid demixing of an athermal mixture of colloids and self-excluding polymers is analysed with the first-order thermodynamic perturbation theory (TPT1) of Wertheim. The colloidal particles and the segments making up the polymer chains are modeled as hard spheres of different diameters. The polydispersity of the polymer is represented by a distribution of chain lengths. The polydispersity increases the extent of demixing; the critical composition remains relatively unaffected, while the critical packing fraction decreases substantially. The polymers with the longest chains are responsible for the increase in the extent of immiscibility. (C) 2004 Elsevier B.V. All rights reserved.