The effect of the SiO₂/Al₂O₃ ratios in ZSM-5 on the gas-phase dehydration of glycerol was examined over Na-ZSM-5 and H-ZSM-5. H-ZSM-5 with a SiO₂/Al₂O₃ ratio of 150 showed the highest glycerol dehydration activity at 315°C among the various ZSM-5 catalysts. H-ZSM-5 with a SiO₂/Al₂O₃ ratio of 30 showed less catalytic activity than H-ZSM-5 with a SiO₂/Al₂O₃ ratio of 150. Several characterization techniques: X-ray diffraction (XRD), the temperature-programmed desorption of ammonia or water (NH₃-TPD, H₂O-TPD) with mass spectroscopy, temperature-programmed oxidation (TPO) with mass spectroscopy, infrared spectroscopy (FT-IR) after pyridine adsorption, and CHNS analysis were employed. The amount of acid sites and the acid strength decreased with increasing SiO₂/Al₂O₃ ratio in the case of H-ZSM-5. The amount of adsorbed H₂O and its binding strength also decreased with increasing SiO₂/Al₂O₃ ratio in the case of H-ZSM-5. Na-ZSM-5, having moderate Lewis acid sites, showed the least catalytic activity, which may be due to its weak acid strength for this reaction and strong adsorption of H₂O on the active sites. As long as the molar ratio between water and glycerol was in the range from 2 to 11, there was no noticeable difference in the glycerol conversion but the acrolein yield increased with increasing fraction of water in the feed.