A rigorous method to determine the adiabatic compressibility of a solvent has been considered. Using the given approach a thermodynamically valid equation was obtained to determine the hydration numbers and adiabatic compressibility of hydrated complexes. Hydration parameters were determined for the chlorides of sodium, potassium, magnesium and calcium, and the sulphates of sodium and magnesium. (c) 2009 Elsevier Ltd. All rights reserved.