Measurements of the viscosity of a suspension of monodisperse polystyrene particles as a function of volume fraction at two fixed values ofκa(3.13 and 8.24) and various values of the electrokinetic (ζ−) potential permit a test of current theories of the primary electroviscous effect. The results reveal a major discrepancy which may be real but which is more likely attributable to the presence of a “gel‐like” layer on the surface of these particles.