we measured the refractive index of nine IL-water mixtures over the entire concentration range from pure water to pure IL. These systems showed slightly convex concentration dependence which differs from the ideal mixing law.Furthermore, plots of the IL concentration dependence of the normalized ( = 0 and = 1) completely overlapped for all ILs studied.Importantly, even though the studied ILs have different combinations of cations and anions the results showed almost the same concentration dependence.Thus, the important point is that knowing the refractive index of pure water and pure IL, the refractive index of an IL-water mixture can be estimated easily for an arbitrary IL concentration using Eq. (3).