A model is developed for predicting the sedimentation velocity in suspensions of multisized nonflocculating solids, in which the retarding effect of the smaller particles on the setting velocities of the larger ones is taken into account. Tests of the model, and comparisons with other models, demonstrate that it provides improved prediction of data on suspensions comprising both discrete particle size mixtures and continuous size distributions, and that it is applicable to continuous countercurrent solid-liquid operations. CONCLUSIONS AND SIGNIFICANCEOperations involving relative motion between a fluid and suspended particles arise often and include sedimentation, fluidization and co-or countercurrent solid-liquid operations.The hydrodynamics of slow vertically flowing liquid-particle mixtures, involving noncolloidal particles of uniform size and density, are understood in the dilute limit and can now be reliably treated empirically in the intermediate and concentrated limits. The presence of particles of mixed sizes enhances in-Correspondence concerning this paper should be addressed to M S. Selim.