Density (rho) and ultrasonic velocity (u) values of amino acid/peptide: L-histidine, L-glutamic acid, L-tryptophan and glycylglycine in 2 M aqueous KCl and 2 M aqueous KNO3 solutions have been measured as a function of amino acid/peptide concentration at different temperatures: 298.15, 303.15, 308.15, 313.15, 318.15 and 323.15 K. Using the rho and u data, the partial molar volumes (phi(v)degrees) and partial molar isentropic compressibilities (phi(k)degrees) have been computed. The increase in partial molar volume with an increase in temperature has been attributed to the volume expansion of hydrated zwitterions. The phi(v)degrees and phi k degrees values of L-histidine, L-glutamic acid. L-tryptophan and glycylglycine in 2 M aqueous KCl and 2 M aqueous KNO3 solutions have been found to be larger than the corresponding values in water. The larger partial molar volumes of L-histidine, L-glutamic acid, L-tryptophan and glycylglycine in 2 M aqueous KCl and 2M aqueous KNO3 solutions have been ascribed to the probable formation of 'zwitterion-K+/Cl-/NO3- and 'k(+)/Cl-/NO3--water dipole' aggregates in solutions. The zwitterions-ions interactions in solutions may cause the release of water associated with zwitterions to the bulk water. The larger partial molar isentropic compressibilities of L-histidine/L-glutamic acid/L-tryptophan/glycylglycine in 2 M aqueous KCl/2 M aqueous KNO3 solutions than the corresponding values in water have been attributed to the zwitterions-ions and ions-waters dipoles interactions in solutions. (C) 2010 Elsevier B.V. All rights reserved.