The unrestricted Hartree-Fock method has been applied to the calculation of theπ-electron spin densities in a number of conjugated carbonyl radicals and anion radicals, the parameters being those previously used in the interpretation of the ionization potentials and electronic spectra of the corresponding closed shell molecules. The results have been compared with measured E.S.R. proton hyperfine coupling constants and, in general, there is a disappointing agreement between theory and experiment. Some reasons for this are suggested.