The viscosity and density of water + sodium chloride + potassium chloride solutions were measured at 298.15 K from very dilute to saturated as well as supersaturated solutions. A 3.5th term in molarity was added to the extended Jones-Dole equation to produce a new equation. This extended Jones-Dole type equation can well represent the viscosities of the systems studied to saturated concentrations. In low concentrations, the calculated viscosity values by simple additivity are close to the experimental values. Above a certain concentration, the calculated viscosities of NaCl + KCl mixtures are lower than the experimental values and the difference becomes greater with increasing concentration.