Refraction index, n D, data of four pure ionic liquids and their solutions with PEG200 were measured at room temperature (298.15 K) and atmospheric pressure (0.1 MPa) over the entire composition range of the mixtures. The data were combined with existing volumetric data in order to yield the molar refraction of all 27 samples. The results show that although the molar refraction values are strongly dependent on the molar volume of the pure components and exhibit ideal behavior for the solutions, the refraction index values show non-linear trends with the solution composition. These findings are interpreted by taking into account the nano-structured nature of ionic liquids, their strong interactions with the glyme-like chains of the PEG molecules, and the relative intensity of the dispersion forces present in all studied systems.