Upon addition of sufficient electrolyte to two poly(n‐butyl methacrylate) latices, differing in particle size by a factor of 11, their viscosities were identical, within experimental error, over the range of volume fraction, φ , from 0.05 to 0.33. The viscosities agreed with those predicted by Mooney’s formula: ln η rel =2.5φ/(1−φ max −1 φ) provided φ max was the random close‐packing value found for monodisperse spheres, 0.637. Upon combining the two latices on an equal weight basis, the viscosities at equal volume fractions were lower than those found for the individual latices. The data, however, fit Mooney's formula provided a larger value of φ max was used.