HZSM-5 used to be the main catalyst component for the MTG process of Exxonmobil Co. Catalyst deactivation by coke in methanol-to-hydrocarbon (MTH) over HZSM-5 was studied in a fixed-bed reactor by observing the activity and selectivity with time-on-stream at 500 degrees C and WHSV=4.25 h(-1). The HZSM-5 with SiO2/Al2O3 ratios of 40 and 280 were synthesized and characterized with XRD. N-2 adsorption/desorption and NH3-TPD. The used catalysts were investigated by N-2 adsorption/desorption, XRD, elemental analysis (EA) and UV-vis spectroscopy to identify the nature of coke. If the acidity of HZSM-5 is low (SiO2/Al2O3=280), the coke deposited is mainly mono- or bi-aromatic which did not affect the catalyst activity significantly. On the other hand when the acidity is high (SiO2/Al2O3=40), the coke deposited is polycyclic aromatics with 3 or 4 fused rings which leads to significant deactivation after 24 h. It is presumed that the coke on the HZSM-5 with low acidity must be located inside the pores and/or in the framework, since the coke particles without polycyclic aromatics should be small. The deactivation of HZSM-5 with high acidity is mainly caused by the bulky coke molecules, most probably inducing the topological blocking of the zeolite pores and channels. (C) 2012 Elsevier Ltd. All rights reserved.