Molecular-orbital calculations of the pi-electron spin densities in a series of aromatic and aliphatic nitrile anion radicals have been performed using the Hückel—LCAO method and the approximate configuration-interaction correction of McLachlan. Coulomb and resonance integrals for the nitrile group were estimated by comparing calculated spin densities with proton and carbon-13 hyperfine splittings obtained from electron-spin resonance measurements. The predicted spin densities were in generally good agreement with the experimental results, but for some of the compounds it is impossible to get an exact fit between theory and experiment within the framework of the simple valence-theory calculations if the sigma—pi interaction parameters relating splittings to spin densities are taken as fixed quantities. Semiempirical treatments of the N14 and C13 splittings in the cyano groups give excellent correlations of the experimental splittings with the calculated spin densities, and estimates have been made of some of the sigma—pi interaction parameters relating to the N14 splitting. Polarographic half-wave potentials have also been compared with calculated pi-electron energies. A discussion is given of the relation between the spin densities predicted by the valencebond and molecular-orbital theories.