The temperature dependence of the proton hyperfine splittings in the E.S.R. spectra of the biacetyl semidione radical CH3C(OH)COCH3is interpreted in terms of the restricted motion about the C-OH bond. An exact analysis of the restricted rotation gives values of the potential barrier of 2·3 to 2·8 kcal mol-1for the solvents dibenzeyl ether, cyclohexene,n-butanol andi-propanol. It is suggested that the main contribution to the potential barrier is the energy gained from delocalization of the electrons into the hydroxyl oxygen orbitals and that this effect is largely independent of the nature of the solvent.