Transient absorption spectroscopy was used to study the mechanism for radiolysis of dense, liquidlike supercritical (SO CO2 (T = 41 degreesC, rho = 0.84 g/cm(3)). The 350-1500 nm spectra are decomposed into. the contributions from the solvent radical cation, solvent radical anion, and a long-lived neutral product that we associate with singlet carbon trioxide, CO3((1)A(1)). These three species are characterized by their optical spectra, chemical behavior, kinetics, and the response of these kinetics to external electric field. The following mechanism for radiolysis Of sc CO2 is suggested: Ionization of the solvent yields 5 pairs per 100 eV. Most of these pairs are comprised of the solvent hole and a thermalized quasifree electron; the prompt yield of CO3- is <3% of the total ion yield. The electrons are trapped by the solvent in <200 ps. Because of high election mobility, most, of this trapping occurs after the charges escape each other's Coulomb field. Because of cross recombination of the electrons with nongeminate solvent holes,. the lifetime of the quasifree electrons is further reduced. A theoretical model that accounts for these dynamics is suggested. It is shown that di- and triatomic molecules donate an electron to the solvent hole, and the resulting solute cations polymerize. Exotic ion species, such as (N2O)(2)(+) and (CO)(n)(+), can be produced this way. Using SC CO2 provides an opportunity to study such multimer cations in liquid solution.