A high pressure, optical system is presented for measuring the refractive index of a supercritical fluid solvent to within±3×10−3at pressures as high as 2500 bar and temperatures of 200 °C. The experimental technique used here is an adaptation of the laser beam displacement method for measuring the refractive index of liquids at ambient pressure. This optical system is used to measure the refractive index of supercritical ethane to pressures of 2000 bar and temperatures of 200 °C. ©1997 American Institute of Physics.