In this paper, we describe an experimental investigation into the dynamic viscous resuspension of bidisperse suspensions in an annular parallel plate device. We use the model of Chapman and Leighton to determine the effective diffusivity of the particulate phase (51.0 and 119.0 mu m glass spheres) in the plane of shear as a function of overall concentration (0.3 < phi < 0.5) as well as the relative size fraction of the large particles (0.0 < phi(L)/(phi(L) + phi(S)) < 1.0). The effective diffusivity of a mixed suspension was found to be somewhat less than the volume average of the diffusivities of monodisperse large and small sphere suspensions.