Phenoxyl radicals were generated in solutions containing from 0 to 75 % sulphuric acid. The changes observed in the e.s.r. spectra over this range are shown to be due to progressive formation of phenol radical cations. In favourable cases plots of the e.s.r. parameters against acidity enabled us to determine the pKa of the radical cations, which varied from –2.0 (phenol radical cation) to –0.8 (hydroquinone radical cation). Changes in the coupling constants in going from a phenoxyl radical to the corresponding phenol radical cation are usually relatively small, but the g-values change from 2.004 4 ± 0.000 4 to 2.003 2 ± 0.000 4. The free energy changes involved in competing oxidative processes are discussed and the data support the view that the C—O group in phenoxyl resembles that in ketones.