The osmotic coefficient data for aqueous solutions containing 1-n-butyl-3- methylimidazolium ([Bmim]) based amino acid ionic liquids (AAILs) made from glycine, L-alanine, L-valine, and L-leucine are obtained in the concentration range of similar to 0.01 mol.kg(-1) to similar to 0.6 mol.kg(-1) at 298.15 K and at 0.1 MPa using vapor pressure osmometry. Activity coefficients of both components, that is, solute and solvent (water), in Mixture were estimated using experimental osmotic coefficient data and have been further used to obtain the excess and mixing Gibbs free energies for the studied systems. The concentration dependence of osmotic coefficients indicates possible ion-pair formation and hence the inn-pair dissociation constant for all the studied AAILs in aqueous solutions at 298.15 K is reported. The nonelectrolyte contribution due to ion-pair formation in the aqueous AAIL solutions was discussed through estimation of osmotic second virial coefficients for AAILs using the McMillan-Mayer theory of solutions. Application of the Pitzer model to the experimental osmotic coefficient data indicated that anion-anion interactions in aqueous solutions of AAILs are favored over the cation-anion interactions with the increasing alkyl chain length on the anion signifying the hydrophobic effect responsible for attraction between the like-charged species.