This report presents the first observation of an alkyl radical in supercritical CO2 by any magnetic resonance technique. Muoniated ethyl radical has been detected in muon-irradiated supercritical CO2 solutions. In the presence of a low concentration of ethene in supercritical CO2, it is found that the addition of muonium to ethene is the only reaction channel, and that the yield of this process is enhanced compared to conventional solvents. The temperature dependence of the hyperfine coupling constants of the ethyl radical suggests that at a density of 0.3 g/cm(3) both the rotational motion of the methyl group and the electronic structure of the radical are similar to those in the gas phase, and therefore that the local environment around the ethyl radical is similar to the gas phase under these conditions. At higher densities, however, there is a remarkable and unexpected density dependence of the hyperfine coupling constant of the ethyl radical, which has never been observed in any environment. In this regime, the density dependence suggests that supercritical CO2 has a significant effect on the electronic structure of the free radical. Thus, changing the density of CO2 offers a possible means of tuning the radical reactivity. In addition, at a density of close to 0.4 g/cm3, CO2 molecules cluster around the ethyl radical, and this increases the local density around the ethyl radical by a factor of similar to 1.5.