X-band time-resolved electron paramagnetic resonance (TREPR) spectra of three flexible biradicals of varying chain length and structure were obtained in liquid and supercritical carbon dioxide (CO2) Solutions and compared to conventional solvents. For C-16 acyl-alkyl biradical 1a, all average spin exchange interaction between the radical centers, J(avg), was obtained by spectral Simulation using a simple model for spin-correlated radical pairs (SCRPs) and a small amount of T-2 relaxation from a previously established J modulation mechanism. A large solvent effect Oil J(avg), was observed for the first time, varying by almost I order of magnitude from CO2 (J(avg) = -120 +/- 10 MHz) to heavy mineral oil (-11 +/- 3 MHz) for 1a. For C-15 bis(alkyl) biradical 1b, spectra obtained under supercritical conditions are only slightly different from those detected in liquid CO2 but differ from spectra taken in benzene. For C-10 acyl-alkyl biradical 2a, more emissive spin polarization due to S-T- mixing is observed in CO2 than in benzene. These results are discussed in terms of solvent properties such as dielectric constant, viscosity, and specific interactions. Both chain dynamics and changes to the equilibrium distribution of end-to-end distances can alter J,,,, and the observed ratio of S-T-0 to S-T- mixing; however, faster chain dynamics is concluded to be the most likely cause of the observed effects in these systems.