Ultrasonic velocity and density values have been measured for ternary systems (amino acid + salt + water): Lisoleucine, L-proline and L-glutamine in aqueous 0.5M K2SO4 solution used as solvent for several concentrations of amino acids at different temperatures in the range of 303.15 to 323.15K. The ultrasonic velocity values have been found to increase with increase in amino acid concentration and temperature in all the systems. The increase in ultrasound velocity with increase in concentration has been discussed in terms of electrostatic interactions occurring between terminal groups of zwitter-ions (NH4+ and COO-) and K+, SO4-2 ions. The interactions of water dipoles with cations/anions and with zwitter-ions have also been taken into consideration. It has been observed that the ion-zwitter-ion and ion-dipole attractive forces are stronger than those of ion-hydrophobic repulsive forces.