The pulse radiolysis technique has been employed in studying charge-transfer reactions of anionic C2O4- and cationic C2O4+ species in supercritical carbon dioxide (scCO(2)) over a range of reduced densities rho(r) = 0.36-1.5 and at a reduced temperature of T-r = 1.03. The absorption spectrum measured in the visible region with a maximum around 700 nm is assigned to the dimer cation C2O4+. The pressure dependence of charge-transfer reactions was examined using dimethylaniline (DMA), benzoquinone (BQ), and oxygen as charge accepters. The reaction rates of DMA with cations, and BQ with anions are at or near the diffusion-controlled limit. The rates decrease an order of magnitude with increase of pressure. The reaction of C2O4+ With oxygen is much slower with an almost constant rate over the pressure range examined. The measured race constants of electron-transfer reactions are analyzed in terms of the diffusion constants of reactants in scCO(2), and the dependence of measured and theoretical values on the bulk density is discussed.