The Huckel equation used in this study to correlate the experimental activities of dilute alkali metal nitrate solutions up to a molality of about 1.5 mol . kg(-1) contains two parameters being dependent on the electrolyte: B [that is related closely to the ion-size parameter (a*) in the Debye-Huckel equation] and b(1) (this parameter is the coefficient of the linear term with respect to the molality and this coefficient is related to hydration numbers of the ions of the electrolyte). In more concentrated solutions up to a molality of 7 mol . kg(-1), an extended Htickel equation was used, and it contains additionally a quadratic term with respect to the molality and the coefficient of this term is parameter b(2). All parameter values for the Huckel equations of LiNO3, NaNO3, and KNO3 were determined from the isopiestic data measured by Robinson for solutions of these salts against KCl solutions [J. Am. Chem. Soc. 57 (1935) 1165]. In these estimations, the Huckel parameters determined recently for KCl solutions [J. Chem. Eng. Data 54 (2009) 208] were used. The Huckel parameters for RbNO3 and CsNO3 were determined from the reported osmotic coefficients of Robinson [J. Am. Chem. Soc. 59 (1937) 84]. The resulting parameter values were tested with the vapour pressure and isopiestic data existing in the literature for alkali metal nitrate solutions. These data support well the recommended Huckel parameters up to a molality of 7.0 mol . kg(-1) for LiNO3 and NaNO3, up to 4.5 mol . kg(-1) for RbNO3, up to 3.5 mol . kg(-1) for KNO3, and up to 1.4 mol . kg(-1) for CsNO3 solutions. Reliable activity and osmotic coefficients of alkali metal nitrate solutions can, therefore, be calculated by using the new Huckel equations, and they have been tabulated at rounded molalities. The activity and osmotic coefficients obtained from these equations were compared to the values suggested by Robinson and Stokes [Electrolyte Solutions, second ed., Butterworths Scientific Publications, London, 1959], to those calculated by using the Pitzer equations with the parameter values of Pitzer and Mayorga [J. Phys. Chem. 77 (1973) 2300], and to those calculated by using the extended Huckel equation of Hamer and Wu [J. Phys. Chem. Ref. Data 1 (1972) 1047]. (C) 2010 Elsevier Ltd. All rights reserved.