The effects of different cation species on water adsorption on hydrophobic ZSM-5 zeolite are investigated through adsorption isotherm and isobar measurements. The adsorption isobars clearly show that sodium cations behave as stronger water adsorption sites than do protons. Furthermore, the adsorption isotherms of Na- and H-zeolites show such differences at part of the hysteresis loop. This suggests that the cation species in ZSM-5 affect the formation of the first, second, and third hydration shells in the hydrophobic space.