The paramagnetic resonance absorption spectra of chlorine dioxide and the alkali superoxides are described in detail and a theoretical analysis is made of the results in terms of molecular orbital theory. The unpaired electron is associated with the binding orbitals in each case and the variation of g values and hyperfine splitting are used to calculate the details of the energy levels. Variation of the spectra with temperature has also been studied and has revealed an averaging effect on the chlorine hyperfine structure due to the `tumbling' motion of the solvent molecules.