Abstract: An improved process is provided for catalytic isomerization of monocyclic methyl-substituted aromatic hydrocarbon compounds of from 8 to 10 carbon atoms contained in a feedstock also containing ethylbenzene in a reaction zone maintained under conditions such that said isomerization is accomplished in the vapor phase. The improvement in the process comprises a first step wherein the temperature is maintained at a level which provides from about 50 mole percent to about 75 mole percent conversion of said ethylbenzene for a period of time of from about 24 hours to about 96 hours, and a second step wherein the temperature is maintained at a level which provides from about 15 mole percent to less than about 50 mole percent conversion of said ethylbenzene.