Nature, population and acidic properties of the hydroxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicalites have been characterised by the adsorption of NH3, monitored through the combined use of microcalorimetry and IR spectroscopy. The experimental data (enthalpy of adsorption, stretching frequency of the O-H bonds perturbed by the interaction with NH3) have been compared with ab initio computational results (binding energy, red shift of the stretching frequency) and were found to be in good agreement. The presence of rings of mutually interacting silanols has been postulated (on the basis of both experimental and computational results) in the case of chemically very pure, defective silicalites. The dimension of the rings depends on the preparation conditions. A perfect silicalite sample, taken as hydrophobic reference material, was found to be virtually unreactive towards NH3, in that it interacts very scarcely with ammonia, showing a heat of adsorption much lower than the heat of liquefaction of NH3 itself. (C) 2002 Elsevier Science B.V. All rights reserved.