It is shown that the addition, over suitable concentration ranges, of mixtures of (nonadsorbing) sodium poly(styrene sulfonate) and potassium chloride, to dispersions of silica particles in water, can lead to very large changes in the sediment height of the resulting aggregates, reflecting similarly large changes in particle packing density within the aggregates. It can also lead to aggregation rates which are considerably faster than the diffusion-controlled rates (by as much as a factor of 2.5), although this enhancement is reduced as the dispersion particle concentration is reduced. (c) 2007 Elsevier Inc. All rights reserved.