Data on water activity in solutions of series of of 3:1 and 1:3 electrolytes [AlCl3, Al(NO3)3, LaCl3, La(NO3)3, H3PO4, NaH2PO4] have been generalized in the frame of solvation excesses concept. Solvation excess reflects the distibution of the selected structural elements (ions or solvent particles) in the solution. The computed values have demonstrated the opposite contributions of ion assotion and hydratiuon of ions and ion assotiates. Solvation excess concept allows determination of water molecules fraction constituting the excess at ions and their associates. The comprision of results obtained with different choice of model structural units has shown the electrolyte concentration ranges of validity of the respective models.