The shear viscosity of monodisperse and binary mixtures of internally-cross-linked polystyrene latices (microgels) dispersed in bromoform has been studied as a function of the applied shear stress, total microgel concentration, and concentration ratio of large to small microgels. For highly cross-linked monodisperse microgels with hydrodynamic radii of 84 and 141 nm, the limiting viscosities at both zero and infinite shear rates, η0 and η∞, are independent of particle size and exhibit the same scaling with volume fraction as hard-sphere-like dispersions. Also in agreement with experimental data for hard-sphere-like dispersions, the critical stress, defined as the stress at which η= (η0 + η∞)/2, displays a maximum as a function of the microgel volume fraction. For binary mixtures having a constant microgel volume fraction, the viscosity exhibits a minimum as a function of the fraction of small particles, reflecting the more efficient packing due to polydispersity. A semiempirical model of the viscosity of microgel mixtures, based on the scaling of the viscosity with “excluded volume”, is proposed. The excluded volume parameter is defined in terms of the hard-sphere osmotic compressibility for mixtures.