The dependence of the viscosities of highly concentrated bimodal suspensions on the particle size distribution and the relative fractions of the two particle populations is investigated using a parallel‐plate viscometer. For a fixed volume fraction of particles the viscosities of the systems with a bimodal size distribution of solids are less than the viscosity of a suspension with a unimodal size distribution of particulates. The results of two‐dimensional simulations in our previous study [C. Chang and R. L. Powell, J. Fluid Mech.253, 1 (1993)] show excellent agreement with the experimental data when normalized with respect to their respective maximum packing values. In addition, both experimental and simulation results for the transient shear properties of bimodal suspensions of spherical particles are reported. Qualitatively, both show the same trend: a suspension with a larger relative viscosity needs higher strains to reach the final steady state as compared with suspensions having lower relative viscosities.