The viscosity of semidilute bimodal suspensions of sterically stabilized silica particles was measured as a function of the total volume fraction and the size ratio of the dispersed particles, and of the composition of the mixtures. The unimodal components of which the bimodal suspensions were composed showed hard sphere behavior. In the mixtures, a minimum in the viscosity was observed at small size ratios (al/as<2) when plotted against the composition. For larger size ratios a maximum was found. The origin of the viscosity maximum is uncertain but might be explained by depletion forces.