Three different types of aggregates of submicron alumina particles were produced utilising either polymer, high salt (1.0 M), or low salt (0.075 M) aggregation conditions. All three types of aggregates had similar structural properties (mass fractal dimension = 2.0). The typical size (d[4,3]) of the polymer aggregates was 125 microns, the high salt aggregates 12 microns and the low salt aggregates 4 microns. It was found that smaller aggregates produced higher gel points and higher apparent maximum packing fractions. Larger aggregates produced higher shear and compressive yield stresses at all volume fractions. The relative effect that aggregate size and interparticle attraction has on the sediment yield stresses was investigated in the salt system. The salt concentration was adjusted so as to produce different size aggregates with the same final salt concentration and thus same level of inter-particle attraction. The size of the aggregates and the magnitude of the inter-particle attraction were found to have similar contributions to the compressive yield stress over the range of size and attraction investigated.