The aim of this work is to solve the key problems of the green production process for chromium trioxide and to study the thermodynamic properties of corresponding solutions. The refractive indices of quaternary mixtures (chromium trioxide + potassium chromate + potassium dichromate + water) have been measured from 298.15-333.15 K. A molecular thermodynamic model of refractive indices for aqueous solutions of chromium compounds is proposed, based on the author's molecular model for solution structure and an equation for the excess enthalpy of a binary solution obtained from the experimental measurements. The model parameters are confirmed and quantitative equations for the variation of the model parameters with temperature and the molality of potassium dichromate in the mixed solvent are established. The data predicted by the model are in good agreement with the experimental results.