Recently a theoretical treatment (J. Phys. Client. 13 2009, 113, 2398-2404) was developed for predicting the standard state thermodynamic properties Of electrolytes LIP to and beyond the critical temperature of water (1273 K and at Pressures Up to 1000 MPa). In general, the model requires Sufficient data at 298.15 K including the Gibbs free energy of hydration and at two higher temperatures to fix two constants for each electrolyte. This communication describes an extension of this "two constant" theory to thermodynamic properties of polyatomic ions For which no accurate data for the Gibbs free energy of hydration exits at 298.15 K.