Catalytic transformation of 1,2,4-trimethylbenzene has been investigated over two different USY-zeolite based catalysts in a novel riser simulator at different operating conditions. FCC-SY catalyst has low acidity and, FCC-CY contains high acidity. The effect of reaction conditions on the variation of the isomerization to disproportionation products ratio (I/D), distribution of trimethylbenzene (TMB) isomers (1,3,5-to-1,2,3) and values of p-xylene/o-xylene (P/O) ratios are reported on both catalysts. A low acidity catalyst was favored in only two reactions; isomerization and disproportionation. No coke and gases were produced over the low acidity catalyst. On the other hand, high acidity catalyst produces huge amounts of gases and coke which are an indication of paring reactions.