The (CO2-)-C-12-O-16, (CO2-)-C-13-O-16, (CO2-)-C-12-O-17, and (CO2-)-C-12-O-16,17 radical anions have been generated by four independent methods and isolated in neon and argon matrices for detailed ESR (electron spin resonance) investigations, Included with these experimental measurements for the various magnetic parameters of CO2- are high level ab initio calculations (MR SD-CI and others) of the C-13 and O-17 hyperfine A tensors. Some of the calculations included the effects of a 42-atom neon cage on the electronic structure of CO2-. Previous ESR studies of CO2- have been conducted in more perturbing environments, such as ionic crystals, where the close proximity of the counter cation can alter the anion’s properties. A comparison of the earlier measurements in more interactive materials with these theoretical and rare gas matrix results reveals a significantly different distribution of the spin density. The neon magnetic parameters (MHz) for CO2- are g(x)=2.0018, g(y)=1.9964, g(z)=2.0010; for C-13, A(x)=320.4, A(y)=296.1, A(z)=394.5; for O-17, A(x)=-81.6, A(y)=-174.9 and A(z)=-151.8. The argon results are similar to these neon values; isotropic spectra in argon were also observed at elevated temperatures that yielded g(iso) and A(iso) parameters consistent with the low temperature (4 K) anisotropic spectra.