HZSM-5 zeolite was found to be a very stable catalyst for the ethanol transformation into hydrocarbons at 350 degrees C and 30 bar total pressure. It was found to maintain high activity for C3+ hydrocarbons formation with time-on-stream in spite of a near total loss of Bronsted acidity, 92% loss of microporosity and high coke content deposited inside its micropore volume. The same solid, passivated with TEOS was tested in the same conditions and it was found that the treatment slightly improved the catalytic performance of the zeolite, even if similar losses of acidity and microporisty were determined after reaction. This shows that C3+ hydrocarbons' formation does not occur at the external surface. Alkyl aromatic hydrocarbons were found occluded in the zeolite structure after reaction, detected by IR spectroscopy analysis and by CH2Cl2 extraction after solubilization of the structure with HF solution. EPR-CW analysis of both coked samples proved existence of free radicals. This last technique could provide us further enlightening of the ethanol transformation mechanism. (C) 2010 Elsevier B.V. All rights reserved.