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The optical index constants of CO[sub 2] at critical density are measured by terahertz (THz) time-domain spectroscopy (TDS–THz) in the frequency range 0.1–1.5 THz. Since supercritical CO[sub 2] is usually presented as a “universal” solvent, we systematically study the refractive index variations in the vicinity of its critical point along the critical isochore by scanning temperature from 295 to 308 K to explore the liquid, the gas, and the supercritical states. Density changes with temperature result in refractive index variations that are found to fit the Lorenz–Lorentz relationship in the THz frequency range. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]