The shear viscosity of electrostatically stabilized bidisperse aqueous silica suspensions has been studied as a function of shear rate, particle size, total volume fraction of the particles, and volume ratio of small to large particles. The higher limiting relative viscosity (at1000 s−1) of monodisperse suspensions of 0.6, 0.85, and 1.5 μm silica particles was found to be particle size dependent indicating deviation from hard sphere behavior. It is shown that the flow properties of bidisperse suspensions of colloidal size particles is not only affected by the volume (weight) ratio of the two sizes, but the absolute size of the particles and the particle size ratio also play an important role in determining the viscosity of the suspension. Binary mixtures of particles of smaller size ratios show higher viscosities than the suspensions containing larger size ratio particles. This indicates that the viscosity response of suspensions of colloidal size particles is controlled by both hydrodynamic interactions and colloidal forces. The Farris [Trans. Soc. Rheol.12, 281 (1968)] model was found to be inadequate when applied to bidisperse suspension of colloidal size particles. An empirical method is suggested to estimate the viscosity of a bimodal suspension at a given volume fraction of the particles, from the viscosity of monodisperse systems and the viscosity of a bimodal suspension (at a fixed total volume fraction) as a function of the volume ratio of the two particles.