Adsorption/desorption of water in the mesoporous material MCM-41 was observed with neutron diffraction. Results obtained with a H2O/D2O mixture contrast-matched to the host material unambiguously show that the pores are fully hydrated in the presence of excess water. During desorption carried out by air-drying a saturated sample, water in the partially filled pores redistributes and the statistical distribution of water within each pore is characterized by a smooth function that decreases from a maximum around the edge to a minimum at the center.