Abstract: his work investigated the effect of ionic liquid on the physical properties of acetonitrile + water with the refractive indices, which were measured for the ternary system (acetonitrile + water + ［Hmim］Cl (1-hexyl-3-methylimidazolium chloride)) and its binary systems (acetonitrile + water, water + ［Hmim］Cl and acetonitrile + ［Hmim］Cl) at 293.15 K by a refractometer. The excess refractive indices of the mixtures were obtained and the relationship of refractive indices with mole fractions was established with an empirical model. It was shown that the excess refractive indices were positive over the whole range of compositions, and there was a maximum for the refractive indices over the range of the investigated mole fraction in acetonitrile + water and the refractive indices in water + ［Hmim］Cl and acetonitrile + ［Hmim］Cl increased as the mole fraction of the ionic liquid increased. Furthermore, the excess refractive indices for the binary and ternary systems were fitted to the RedlichKister, Cibulka and Singh equations, respectively. As a result, the Kohler equation can successfully predict the refractive indices for the ternary system.