Experimental measurements of density rho, speed of sound u, and viscosity eta of aqueous solutions of various 1-alkyl-3-methylimidazolium based ionic liquid (IL) solutions have been performed in dilute concentration regime at 298.15 K to get insight into hydration behaviour of ILs. The investigated ILs are based on 1-alkyl-3-methylimidazolium cation, [C(n)mim] having [BF4](-), [Cl](-), [C1OSO3](-), and [C8OSO3](-) as anions where n = 4 or 8. Several thermodynamic parameters like apparent molar volume phi(V), isentropic compressibility beta(s), and viscosity B-coefficients have been derived from experimental data. Limiting value of apparent molar volume has been discussed in terms of intrinsic molar volume (V-int) molar electrostriction volume (V-elec). molar disordered (V-dis), and cage volume (V-cage). Viscosity B-coefficients have been used to quantify the kosmotropic or chaotropic nature of ILs. Hydration number of ILs obtained using elctrostriction volume, isentropic compressibility, viscosity, and differential scanning calorimetry have been found to be comparative within the experimental error. The hydrophobic hydration has found to play an important role in hydration of ILs as compared to hydration due to hydrogen bonding and electrostriction. Limiting molar properties, hydration numbers, and B-coefficients have been discussed in terms of alkyl chain length of cation or nature of anion. (c) 2011 Elsevier Ltd. All rights reserved.