Abstract: A substituted diaryl compound or a mixture of substituted diaryl compounds, such as dimethylbiaryl compounds are isomerized by treatment with a strong acid or mixtures of strong acids in the absence of any additional catalyst. The strong acid or mixture of acids has a Hammett acidity of less than about −12.6. The isomerization reaction conditions such as reaction temperature, amount of acid, and amount of solvent can be adjusted to selectively produce desired isomers in high yields. One or more desired isomers may be produced in high yields by isomerization of substituted diaryl compounds and selectively removing one or more desired isomers from the resulting equilibrium mixture of isomers. The isomer mixture may be re-isomerized subsequent to selective removal of the desired isomer to produce additional amounts of desired isomer.