Monodisperse electrostatically stabilized polymer latices with particles diameters of 310 and 120 nm were synthesized and used to prepare binary mixtures with bimodal size distribution. The zero shear viscosity of these bimodal, charge stabilized polymer latices was studied at different salt concentrations. In contrast to hard sphere colloidal suspensions, no minimum in viscosity was found as a function of the mixing ratio of small and large particles. Instead the viscosity increased when the fraction of small particles increased, which is due to the direct Coulomb interaction. In order to compare the results with data from hard sphere systems, we used an effective volume fraction phi (eff). The experimentally determined volume fraction at the divergence of the zero shear viscosity is compared with the hard sphere value in order to define phi (eff). The effective volume of the particles was then used to calculate the effective volume fraction of the binary mixtures containing small and large particles. When using phi (eff), a minimum of the viscosity was found at a composition of similar to 30% of small particles similar to the behavior of bimodal hard-sphere suspensions. The volume fraction at maximum packing could be calculated by theoretical models only at high salt concentration. The models underestimate the maximum volume fraction at low salt concentration.