The electrochemical and electron spin resonance (ESR) fingerprints of six isomers of C-84 [D-2(IV), D-2d(II). D-2d(I), D-2(II), C-2(IV), and C-s(b)] are presented together with density functional theory (DFT) calculations of the molecular orbitals (MO) energy levels for a total of 10 isomers (D-2(IV), D-2d(II), D-2d(I), D-2(II), C-s(V), and Cr(I) to C-s(V)). Comparison between calculations and electrochemical data shows a true synergy between calculated energy levels and experimental redox potentials. Assignments are proposed of the C-2 isomer as C-2(IV) and tentatively of the C-s(b) isomer as C-s(V). The temperature-dependent ESR spectra of D-2(IV) and D-2d(II) singly charged ions show an abrupt change around 150 K which is ascribed to a change of spin state. Additionally, the room-temperature solution ESR spectra of all isomers studied exhibit a fine structure characteristic of each isomer.