In this work the primary electroviscous effect was determined for two polystyrene latexes with different surface charge densities and functional groups (sulfonate and carboxylate). For each latex, the electroviscous coefficient (p) was obtained for several electrolyte (NaCl) concentrations; that is, as a function of the electrokinetic radius Ka. In both cases, a maximum in the p against Ka plot was found experimentally, which was not predicted by the theory. A study using Booth's and Watterson-White's theories was done to explain the initial differences between experimental and theoretical data at low Ka values. These maximum values were theoretically explained as a consequence of a third ionic species present in the solution.