In this paper, a theoretical formula on the frequency dependence of the refractive index of a solvent mixture was introduced and the result of the certification of the theoretical formula was reported on several organic solvent mixtures. The difference between the theoretical value and the experimental one was also examined from the view-point of molecular interaction. In the case of cyclohexane-dioxane mixture, the above difference was scarecely observed, but in the case of benzene-cyclohexane mixture, it always showed a considerably large order of deviation and this fact gave us a speculation for the formation of a molecular complex.