A fast kinetic technique using high energy electron beams was applied to the study of reactions of radical ions in supercritical water and carbon dioxide. The high energy electron beams can easily ionize water molecules and produce hydrated electrons, OH radicals and H atoms. The reaction rate between the hydrated electrons and negative ions decreased with increasing temperature because of the repulsive interaction. On the other hand, the reaction rate between the hydrated electron and hydronium ions increased with temperature because of the attractive interaction. In supercritical CO
2, dimer anions (CO
2)
2- and dimer cations (CO
2)
2+ were produced by ionization of CO
2. Transient visible absorption spectra of these species were measured. Comparison with the photobleaching of CO
2 anion clusters in solid rare gas matrices and their reactivity with H
2 and O
2 confirms the assignment. It is well established that the adiabatic electron affinity of CO
2 is negative, but the adiabatic electron affinity of CO
2 dimer has been calculated to be 0.89 eV for D
2d symmetry (CO
2)
2- in gas phase.
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