A reaction network for the conversion of 1,2,4-trimethylbenzene [1,2,4-TMB] over HZSM-5 is proposed, based on product concentrations versus space time studies at 723 K. At conversion levels below 10% the rates of formation of 1,2,3- and 1,3,5-TMB reflect the catalyst's activity for the isomerization of 1,2,4-TMB; the isomer distributions in the xylene and tetramethylbenzene fraction are not thermodynamically controlled and thus represent valuable information on the selectivity of the catalyst. As expected, product distributions were affected by the level of conversion, and thus selectivities have to be compared at equal conversions.