Ultrasonic velocity and density values Of L-histidine or L-glutamic acid or L-tryptophan or glycylglycine + 2 mol.L-1 aqueous KCl or 2 mol.L-1 aqueous KNO3 Solution have been measured as functions of amino acid or dipeptide concentration and temperature from T = (298.15 to 323.15) K. Using ultrasonic velocity and density data, the thermodynamic parameters isentropic compressibility (K-s) and change in isentropic compressibility values have been computed. The K-s values decrease with an increase in the amino acid/dipeptide concentration as well as with temperature of the systems tinder investigation. The decrease in K, values with an increase in concentration of amino acid/dipeptide in aqueous solutions of KCl and KNO3 has been ascribed to an increase in the number of incompressible zwitterions in the solutions.