Thermal electrons attach reversibly to CO2 in supercritical ethane. The equilibrium constants range from 40 to 2 x 10(4) m(-1) for pressures from 50 to 220 bar at 33 and 37 degrees C. The attachment rate increases and the detachment rate decreases as the pressure increases. The lifetime of CO2- is between 20 and 100 ns in the high-pressure range. The reaction volume changes more than an order of magnitude, from -20.0 to -0.5 L/mol over the pressure and temperature ranges studied. The activation volume is approximately 50% of the reaction volume. Electrostriction volumes of the CO2- ion, calculated by a compressible continuum model, account for the main part of the observed reaction volumes. The calculation shows that the high-density region around each CO2- extends to 1 nm.