Aiming at the evaluation of the impact of the ionic liquids (ILs) cation symmetry on their phase behaviour, in this work, novel mutual solubilities with water of the symmetric series of [C(n)C(n)im][NTf2] (with n=1-5) were determined and compared with their isomeric forms of the asymmetric [C(n)C(1)im][NTf2] group. While the solubility of isomeric ILs in water was found to be similar, the solubility of water in ILs follows the same trend up to a maximum cation alkyl side chain length. For n >= 4 in [C(n)C(n)im][NTf2] the solubility of water in the asymmetric ILs is slightly higher than that observed in the symmetric counterparts. The thermodynamic properties of solution and solvation derived from the experimental solubility data of ILs in water at infinite dilution, namely the Gibbs energy, enthalpy and entropy were used to evaluate the cation symmetry effect on the ILs solvation. It is shown that the solubility of ILs in water is entropically driven and highly influenced by the cation size. Accordingly, it was found that the ILs solubility in water of both symmetric and asymmetric series depends on their molecular volume. Based on these findings, a linear correlation between the logarithm of the solubility of ILs in water and their molar volume is here proposed for the [NTf2]-based ILs at a fixed temperature. ; This work was financed by national funding from Fundac¸ ão paraa Ciência e a Tecnologia (FCT, Portugal), European Union, QREN,FEDER and COMPETE by the projects PEST-C/CTM/LA0011/2013and PEST-C/EQB/LA0020/2013. The authors also thank FCT forthe PhD and postdoctoral grants SFRH/BD/87084/2012, SFRH/BD/70641/2010, SFRH/BPD/88101/2012 of M.A.R.M., C.M.S.S.N., andK.A.K., respectively.