The primary electroviscous effect and ζ potentials in two different series of polystyrene lattices of increasing particle size, prepared without addition of surfactants, have been measured, in the presence of increasing concentrations of quaternary ammonium counterions of different ionic sizes. It has been found that whereas for the larger particle size lattices the measured electrical contribution to the primary electroviscous effect (ηel/η0φ) = (η - η0)/η0φ - 2.5) decreases with increasing counterion concentration, as required by the theory, it either remains more or less constant or increases slowly in the case of smaller particle size lattices due possibly to the simultaneous occurrence of a slow feeble agglomeration of the latex particles in the presence of increasing counterion concentrations. Stability calculations according to the DLVO theory tend to support this conjecture. The electrophoretically measured ζ potentials show a steady decrease with increasing counterion concentration; a Langmuir-type isotherm relating the decrease of charge density in the Stem plane with the added counterion concentration has been found to be mostly able to account for the observed results. © 1986.