The activity of a H-ZSM5-90 for the dehydration of ethanol at 473 K increased with an increase in time on stream. The results of NH3-TPD and Ar adsorption showed that the number of acid sites of the H-ZSM5-90 increased during the dehydration. On the other hand, the activity and the number of acid sites of H-mordenite-90 were almost unchanged during the dehydration. Since the pre-treatment of the H-ZSM5-90 with ethylene at 473 K largely increased its activity for the dehydration of ethanol, it is suggested that a large number of new acid sites could be formed inside the pores of H-ZSM5-90 via the polymerization of ethylene produced by the dehydration of ethanol.