All the proton hyperfine splittings in the E.S.R. spectrum of the biacetyl semidione radical CH3Ċ(OH)COCH3are temperature and solvent dependent. These effects are a consequence of the restricted motion about the Ċ-OH bond which causes changes in the electron spin density throughout the radical. McLachlan S.C.F. spin density calculations successfully predict the changes in the three proton hyperfine splittings and, combined with the observed methyl proton splittings give a constant value forQCCH3Hequal to 25·0 ± 0·1 gauss for the solventsi-propanol,n-butanol, cyclohexane and dibenzyl ether from 250 to 400 K.