The data obtained from density (ρ) and viscosity (η) measurements is used for the determination of apparent molar volume (ϕ_V), viscosity B-coefficient of _L-tryptophan in aqueous solution of alkali halides at temperatures T=298.15 K- 313.15 K under atmospheric pressure. From the apparent molar volume (ϕ_V) calculated from experimental density, the equation suggested by Masson is used for calculation of the limiting apparent molar volumes (ϕ_V⁰) and experimental slopes (S_V^*) and its values are used to interplay solute–solvent and solute–solute interactions. The free energies of activation per mole of solvent (Δμ₁^0≠) and solute (Δμ₂^0≠) were calculated. The limiting partial molar volumes of transfer and the free energies of activation per mole of both solvent (Δμ₁^0≠) and solute (Δμ₂^0≠) were analyzed constructed on the transition state theory and cosphere overlap model, respectively. The hydration number of _L-tryptophan in aqueous solution of LiCl, NaCl and KCl was determined using ϕ_V⁰and viscosity B-coefficient.