Eight well-known equations presented for thermodynamic activities in aqueous NaCl and KCl solutions at 298.15 K were tested with the most reliable experimental results of the isopiestic determinations reported in the literature. Usually the equations tested do not predict these data very well. Only the two-parameter Pan equation with the parameter values determined recently explains these data within experimental error up to a molality of 2.0 mol.kg-1. The isopiestic results can also be predicted almost within experimental error up to the molality of the saturated KCl solution by means of a new three-parameter Glueckauf equation. The recommended values of the isopiestic ratio for this electrolyte pair have been tabulated at rounded molalities on the basis of the equations mentioned above. In addition, it was shown that activities in NaCl and KCl solutions are known considerably less accurately above than below a molality of 2.0 mol.kg-1.