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.