Xylene transformation reactions have been investigated over ZSM-5 zeolite in a riser simulator that mimics closely the operation of commercial fluidized-bed reactors. Two reaction schemes have been used to model the transformation reactions. The first is based on a triangular reaction path which assumes a direct interconversion between o- and p-xylene isomers (1,3-methyl shift), and a second scheme assumes the reactions proceed via 1,2-methyl shift only (o-xylene reversible arrow m-xylene reversible arrow p-xylene). The rate constants and activation energies are obtained from simplified kinetic models based on the isomerization of the pure xylene isomers using the "time on stream" decay model. The results provide ample evidence to suggest that direct interconversion between o- and p-xylene isomers (a 1,3-methyl shift) occurs with the same rate as the conversion of m-to o-xylene (a 1,2-methyl shift) over ZSM-5 zeolite catalyst, and the 1,3-methyl shift reaction path is a better representation of the xylene isomerization mechanism in ZSM-5 zeolite than that of the 1,2-methyl shifts only. In addition, the riser simulator and the modeling procedures employed have shown to be very effective in investigating xylene isomerization kinetics.