The origin of the difference in the heat of immersion in water between amorphous and crystalline silicas has been studied. We have investigated the effect of crystallinity of silicas on the heat of immersion. The degree of crystallinity was controlled by heating a amorphous silica with sodium chloride appropriate time. The heat of immersion closely depended on the crystallinity. The IR spectra of surface hydroxyl groups revealed that the increase in the heat of immersion with the crystallinity was attributed to the increase in hydrogen-bounded hydroxyl groups. It is concluded that a regular arrangement of the hydrogen-bounded hydroxyl groups on the crystalline silica surfaces stabilizes adsorbed water layer extensively and causes the increase in the heat of immersion compared to that of amorphous silicas.