A good linear relationship between the concentrations of binary organic liquid mixtures and their refractive index was observed, however, we found that there was a non-ideal behavior of refractive index on aqueous solutions. In many occasions, the refractive index of the mixtures was higher than that of pure organic liquid used. The deviations were greater as the refractive index difference (ΔRI) between water and the other liquid were smaller, however, an ideal behavior was observed for the aqueous solution of ethylene glycol which had the largest ΔRI among the solutions investigated.