Densities of L-alanine/L-glutamine/glycylglycine + 0.512 mol.kg(-1) aqueous K2SO4/KNO3 systems have been measured for several molal concentrations of amino acids/peptide at different temperatures from T = (298.15 to 323.15) K using a density and sound velocity meter (Anton Paar DSA, 5000 m). Using the density data the partial molar volumes (phi(0)(v)) and transfer partial molar volumes (Delta(tr)phi(0)(v)) have been calculated. The measured and calculated thermodynamic parameters have been discussed in terms of zwitterion-ion, zwitterion-water dipole, ion-water dipole, and ion-ion interactions operative in the systems.