The Huckel equation used ill this Study to correlate the experimental activities of dilute RbCl and CsCl solutions up to a molality of about 3.5 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 LIP to the saturated molality of RbCl (= 7.78 mol . kg(-1)) and LIP to I molality of about 8 mol.kg(-1) for CsCl, all extended Huckel equation was used. 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 RbCl were determined from the isopiestic data measured by Rard for NaCl and RbCl solution, (J. Chem. Eng. Data 1984, 29, 443-450) and all parameters for CsCl from the isopiestic data measured by Rard and Miller for NaCl and CsCl Solutions (J. Chem. Eng. Data 1982, 27, 169-173). In these estimations, the Huckel parameters determined recently for NaCl Solutions (J. Chem. Eng. Data 2009, 54, 209-219) were used. The resulting parameter values were tested with the cell potential, vapor pressure, and isopiestic data existing in the literature For RbCl and CsCl Solutions. Most of these data call be reproduced within experimental error by means of the extended Huckel equation LIP to a molality of about 8.0 mol.kg(-1) . Reliable activity and osmotic coefficients for RbCl and CsCl Solutions call, therefore, be calculated by using the new Huckel equations, and they have been tabulated here at rounded molalities. The activity and osmotic coefficients obtained from these equations were compared to the values suggested by Rard (100, see citation above), Rard and Miller (CsCl, see citation above), Land Robinson and Stokes (Electrolyte Solutions, 2nd ed.; Butterworths Scientific Publications: London, 1959). These Values were also compared to those calculated by using the Pitzer equations with the parameters of Pitzer and Mayorga (J. Phys. Chem. 1973, 77, 2300-2308) and Pitzer (Activity Coefficients in Electrolyte Solutions, 2nd ed.; CRC Press: Boca Raton, 2000; pp 100-101) and to those calculated by using the extended Huckel equation of Hamer and Wu (J. Phys. Chem. Ref. Data 1972, 1, 1047-1099).