Adsorption of water on M41s systems has been studied by means of microcalorimetry and IR spectroscopy. After thermal treatment at only 423 K, the samples show two types of surface patches, one hydrophobic (characterized by isolated silanols not interacting with water) and the other highly hydrophilic (showing the highest heat of reversible adsorption observed so far for the H2O/SiO2 system). Thermal treatment at 1023 K causes an extensive dehydration, partially recovered by dissociative adsorption. Both the propensity for silanol condensation (and the opposite reaction of hydrolysis of siloxane bridges) and the very high heat of reversible adsorption are related to the structural properties of M41s.