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 f
or the isomerization of 1,2,4-TMB; the isomer distributions in the xyl
ene and tetramethylbenzene fraction are not thermodynamically controll
ed and thus represent valuable information on the selectivity of the c
atalyst. As expected, product distributions were affected by the level
of conversion, and thus selectivities have to be compared at equal co
nversions. (C) 1998 Academic Press.