The optical heterodyne detected (OHD) birefringence of CO 2 gas in the reduced density range from r/r c = 0.09-0.82 and in the room temperature liquid are reported. Each birefringent response is separated into an electronic, rotational diffusion and non-diffusive nuclear component. The CO 2 rotational relaxation times in the gas phase scale inversely with density in accordance with extended diffusion models of rotational reorientation. The rotational diffusion component of CO 2 liquid (t D = 0.34 ps) appears to obey Debye-Stokes-Einstein behavior. The non-diffusive nuclear responses of the vapor samples exhibit only very modest changes with density and closely resemble the response function of the classical linear free rotor. The dominant, but not exclusive, contribution to the decay of the Raman anisotropy of high-pressure CO 2 gas is inertial motion. The spectral density of the non-diffusive nuclear response of liquid CO 2 is broader and has higher frequency components than the corresponding gas-phase spectral densities, indicating the relative importance of interaction induced effects. The greater extent of collision-induced effects is clearly evident for CS 2 liquid compared with liquid CO 2 when the spectral densities of the non-diffusive nuclear responses of liquid CO 2 and CS 2 are compared as a function of reduced frequency. Inertial motion clearly makes a much larger contribution to the birefringence of liquid CO 2 than to that of liquid CS 2 (at room temperature).