Ultrasonic velocity of liquids and solutions are a simpler but accurate method of analyzing the interaction between the molecules. The passage of ultrasonic waves through solutions and liquids disturbs the equilibrium between the solute and solvent molecules. The velocity of such a wave is a thermodynamic quantity and it is related to the physico-chemical properties of the medium. Ultrasonic velocity (U), density (ρ) and viscosity (η) of Diammonium phosphate fertilizer in aqueous media have been measured at 303, 308, 313, 318, and 323 0 K to estimate hydration number and its allied parameters like apparent molal volume (φv), apparent molal compressibility (φk), molar solvated volume (φs), limiting apparent molal compressibility (φk 0), and limiting apparent molal volume (φv 0) and their constants Sk and Sv. The datas have been interpreted in terms of solute - solvent interactions.