Effect of particle size and polydispersity on the viscosity and maximum packing fraction of aqueous colloidal dispersions has been studied. For dispersions of mono-sized particles, the results indicate that there is a linear relationship between the log(η) (viscosity) and particle size at a fixed shear rate and volume fraction of solids. However, there is a particle diameter at which there is a decrease in the dependency of viscosity on particle size as the slope of the linear plots of log(η) versus particle diameter changes to a smaller value. Preliminary calculations indicate that this particle size may correspond to a separation distance at which electrostatic energy as compared with the thermal energy of the particles can be ignored. In the case of bimodal dispersions, the viscosity is affected by both absolute size and the ratio of the two sizes. The effect of particle size ratio on the viscosity was investigated using bimodal dispersions of the same size coarse particles, but fines of different sizes. There is a critical volume ratio below which bimodal dispersions of larger size ratios show lower viscosities than systems of smaller size ratios. Above this volume ratio of the two sizes, the trend becomes reversed and the fines will have a dominant effect on the viscosity behavior of the bimodal system. Statistically designed experiments were carried out using trimodal mixtures of monodispersed silica particles and it was shown that tridispersed suspensions demonstrate similar behavior as bidispersed suspensions, with a minimum in viscosity observed as a function of particle volume ratio.