In this paper, we investigate the dependence of the viscosity of vibrated bidisperse granular suspensions of spherical particles on the relative fraction of the two populations. At the same total volume fraction I center dot, the viscosity of the bidisperse suspension is lower than that of a monodisperse suspension. A unified viscosity model is derived using an effective free volume approach based on the definition of an equivalent mean diameter and a maximum packing fraction, both depending on the fraction of large particles xi. The resulting model accounts for the viscosity of the suspending fluid eta (f) , the amplitude A, and the frequency f of the vibrations, for a constant ratio of the diameters of the beads lambda = 5.3 and a fixed value of the total volume fraction I center dot a parts per thousand 0.61. It led to a general equation describing both the rheology of monodisperse and bidisperse suspensions.