Measured values of ultrasonic velocity and density have been used to calculate the isothermal compressibilities (kappa(T)) of L-histidine/L-glutamic acid/L-tryptophan/glycylglycine + 2 mol L-1 aqueous KCl/KNO3 systems as a function of amino acid/di-peptide molal concentration and at temperatures in the range of 298.15-323.15 K. The trends of variations of kappa(T), on successive addition of amino acid/di-peptide amount to solutions and on increase in temperature have been discussed in terms of solute-solute and solute-solvent interactions operative in the studied systems as well as difference in interactions with KCl/KNO3 at same concentration have been shown. (C) 2012 Elsevier B.V. All rights reserved.