The catalytic ethylation of ethylbenzene with ethanol over ZSM-5 based catalyst has been investigated in a riser simulator that closely mimics the operation of fluidized-bed reactors. Experimental runs were conducted with a constant ethylbenzene to ethanol molar ratio of 1: 1 over the temperature range of 300-500 degrees C and reaction times of 3-15 s. The effect of reaction conditions on the variation of ethylbenzene conversion, distribution of diethylbenzene isomers, and values of benzene/diethyl benzene ratios are reported. The results show that at low temperatures (300-400 degrees C alkylation is the predominant reaction while at higher temperatures other competing reactions such as disproportionation and cracking are also important. On the basis of these observations, two mechanisms (one for low temperatures and the other for both low and high temperatures) were proposed to represent the ethylation of ethylbenzene with ethanol. The experimental results were modeled using a quasi-steady state approximation with catalyst deactivation function based on a time oil stream (TOS) C, model. Kinetic parameters for ethylation of ethylbenzene with ethanol, disproportionation of ethylbenzene, and cracking of diethylbenzene were estimated by nonlinear regression analysis. The apparent activation energies were found to increase as follows: E-e, < E-d < E-c.