In-line analysis of refractive index is required for efficient design and monitoring of supercritical fluid extraction and precipitation processes. In the present work, a robust method has been developed based on measurements of laser beam deviation using an interferometer and image processing system. Data on refractive index of CO2-ethanol mixtures were obtained at pressures between 70 and 200 bar and temperatures between 308 and 363 K, for continuous flow of premixed solvents and, in addition, for equilibrium gas phase below the mixture critical pressure. The refractive index of a mixture is a linear function of ethanol mole fraction and can adequately describe mixing and phase behavior in the vessel. For pure CO2, refractive index was determined as a function of pressure and density and its Lorentz-Lorenz functions determined.