In this paper, we present experimental measurements for the dynamic viscosity of macroscopic (non-Brownian and noncolloidal) suspensions of bimodal sized spheres when submitted to an oscillating plane Couette flow. The measured viscosity is what we call the dynamic viscosity at finite frequency. Concerning the viscosity of such systems, numerous experimental studies have been done under steady flow conditions, i.e., at zero frequency, but few studies concern the dynamic case. Our measurements have been performed for different values of the three relevant parameters, namely the size ratio λ, the fraction ξ of small spheres to total solids, and the total solid volume fraction Φ. Our results show a viscosity reduction upon mixing, which increases as the total solid volume fraction Φ is increased. We analyze our results by a model that takes into account the volume fraction Φ and the maximum volume fraction Φm, which depends on the two parameters λ and ξ. On the other hand, we compare our experimental results with recent numerical simulations performed by Chang and Powell [J. Fluid Mech. 253, 1–25 (1993); Phys. Fluids 6, 1628–1636 (1994)] by Stokesian dynamics, and Monte Carlo method, which lead, respectively, to viscosity at zero and infinite frequency. Our experimental results lie between these two different simulation results. © 1997 Society of Rheology.