The dehydration process, induced by thermal treatment, of a silica gel, obtained from colloidal nano-particles, has been investigated. At sufficiently low water contents in the structure; water molecules wet the hydrophilic interior surface of the pores rather than forming droplets of pore water. This is manifested by elongation of the OH-bonds of the acidic silica groups and a dramatic downshift in the OH-vibrational frequency is observed. Further dehydration results in a dry. partially hydroxylated, porous silica consisting of a three dimensional network SiO2 nano-particles. Upon further thermal treatment the remaining hydroxyl groups condense to Si-O-Si bridges between the colloidal particles. (C) 2003 Elsevier B.V. All rights reserved.