A process is described for converting at least one isomer of a dialkyl-substituted biphenyl compound, such as at least one 2,X′ dialkylbiphenyl isomer (where X′ is 2′, 3′ and/or 4′), into at least one different isomer, 3,3′, 3,4′ and/or 4,4′ dialkylbiphenyl isomer. The process comprises contacting a feed comprising the dialkyl-substituted biphenyl compound isomer with an acid catalyst under isomerization conditions.