An experimental study of the thickening and filtration characteristics of hematite suspensions consisting of closely sized particles has been undertaken with a view to investigating the influence of polyelectrolytes on these processes. The results show that a certain optimal polymer addition exists for both processes after which no further beneficial effects are observed. This polymer concentration is much lower for filtration than for thickening. The origins of the maximum occurring in the filtration rate with increasing polymer dosage is discussed in terms of polymer adsorption. It is concluded from filtration experiments with coarse de-slimed particles, and also a sintered disc, that the fall in filtration rate at polymer concentrations above the optimum value occurs because of increasing bed structure and not, as was formerly supposed, because of particle restabilization.