In this Letter the results of compressibility measurements made on 1: 1 electrolytes containing normal aliphatic chains (monoalkylammonium salts and those of normal aliphatic acids) in water and aqueous HCl and NaOH solutions at 25 degrees C have been used to calculate the hydration numbers of ions. It was shown that they depend mainly on the ionic size. The idea of additivity of the hydration number n(h) with the constituents of solute molecule was successfully applied. The contributions from -NH3+ and -COO- groups to the total nh of salts were determined. (C) 2008 Elsevier B. V. All rights reserved.