The naphthazarin semiquinone cation in sulphuric acid exhibits an electron spin resonance spectrum which varies with the temperature. This variation is ascribed to rotational isomerization and the shape of the spectrum at different temperatures is reconstructed using a theory based on the Bloch equations. From the temperature dependence of the rate of isomerization, the potential barrier to rotation is estimated to be 4 kcals. The mechanism of isomerization probably involves the breaking of an intramolecular hydrogen bond.