The thermodynamic properties of aqueous solutions of HCl, LiCl, NaCl, KCl, CsCl, NH4Cl, and NaBr between 273 and 523 K and up to m = 1 mol kg(-1) can be described by an extended Debye-Huckel theory which incorporates the hydration of the ions by ligand-binding equilibria. The purpose of this model is to find out some measure of the thermodynamics of the hydration spheres. The following conclusions can be drawn : (i) Whereas the structure formation in binary solutions with HCl and LiCl decreases monotonically with increasing temperature, the other electrolytes change at about 340-380 K from structure breakers to structure formers. This is indicated by the enthalpy/entropy of transfer of water from the bulk to the hydration spheres of the ions. (ii) Beyond about 500 K, HCl is incompletely dissociated. This can be inferred from the relation of the distance of closest approach to the sum of the ionic radii. By the same criterion CsCl is a weak electrolyte in the complete temperature range. (iii) Activity and osmotic coefficients of ternary solutions of these electrolytes can be calculated without additional mixing parameters in the same temperature and molality ranges as the binaries with good accuracy, i.e., with deviations below 1% and 0.5%, respectively. Exceptions are the solutions with the weak CsCl. (iv) Ternary enthalpies can be predicted with deviations of about 35 J mol(-1). (v) Between 298 and 373 K the results also hold true for the bromides.