Bidisperse suspensions of latex beads with a discrete distribution were investigated in the crossflow microfiltration process. The critical fluxes of single-sized latex particles (3.0, 5.0 and 10.0 mu m) and mixed suspensions (3.0 + 5.0, 5.0 + 10.0 and 3.0 + 10.0 mu m) were determined using a non-invasive, in situ, direct observation through the membrane (DOTM) technique. While the critical fluxes of the single-sized particles were consistent with past studies in the literature, the critical fluxes of the mixed suspensions were found to increase in the presence of the bigger particles. In addition, only the smaller size particles deposited on the membrane surface even when the imposed flux was higher than the critical fluxes for the combined and individual components in the bidisperse system. The most plausible explanation is that the enhancement of critical flux is due to augmentation of shear-induced diffusivity of the smaller particles by the bigger particles. (c) 2006 Elsevier B.V. All rights reserved.