The Huckel equation is used to correlate the experimental activities of dilute NaCl and KCl solutions up to a molality of about 1.0 mol . kg(-1). The two parameters of this equation are dependent on the electrolyte and are B [which is simply related to the ion-size parameter (a*) in the Debye-Huckel equation] and b, (which is the coefficient of the linear term in molality and related to hydration numbers of the ions of the electrolyte). In more concentrated solutions up the saturation for both electrolytes, an extended Huckel equation was used, and it contains, additionally, a quadratic term with respect to the molality with parameter b(2). The values of parameters B and b, for dilute KCl solutions were determined from the cell potential differences measured by Hornibrook et al. on concentration cells with transference (J. Am. Chem. Soc. 1942, 64, 513-516). With these values of KCl, the corresponding parameter values for NaCl solutions were determined from the isopiestic data of Robinson for NaCl and KCl solutions (Trans. R. Soc., N. Z, 1945, 75, 203-217). Only the data points for NaCl molalities less than 1.4 mol . kg(-1) were included in this determination. The resulting parameter values were successfully tested with all reliable cell potential and isopiestic data in the literature for dilute NaCl and KCl solutions. For more concentrated solutions, new values of parameters b, and b2 were determined for the extended Huckel equations of NaCl and KCl, but the same values of parameter B were used as for dilute solutions. For these more concentrated NaCl solutions, the values of parameters b, and b2 were determined from the vapor pressure data of Olynyk and Gordon (J. Am. Chem. Soc. 1943, 65, 224-226), which cover the molality range (2.3 to 6.1) mol.kg(-1). With these values for NaCl, the corresponding parameter values for more concentrated KCl solutions were determined from the isopiestic data of Robinson for this pair of electrolytes (see the citation above), where all experimental points were included in the determination. The resulting extended Huckel equations were thoroughly tested with all reliable experimental data presented in the literature on the basis of electrochemical, isopiestic, and direct vapor pressure measurements. Most of these data can be reproduced within experimental error by means of the extended Huckel equations up to the saturated solutions. Reliable activity and osmotic coefficients of NaCl and KCl can, therefore, be calculated by using the Huckel and extended Huckel equations determined in this study. The values obtained by these equations are probably the most accurate values available, and they have been tabulated at rounded molalities. The activity and osmotic coefficients obtained from the new equations were compared with the values suggested by Robinson (see citation above), with those calculated by using the Pitzer equations of Pitzer and Mayorga (J. Phys. Chem. 1973, 77, 2300-2308), with those calculated by using the extended Hiickel equations of Hamer and Wu (J. Phys. Chem. Ref. Data 1972, 1, 1047-1099), and with those calculated by Huckel equations of Partanen and Minkkinen (Acta Chem. Scand 1993, 47, 768-776).