The structural properties of alumina–sodium–silicate fiberglass (S) and microporous silica–alumina fiberglass supports (CCB18 and CCB20) were investigated using solid state NMR. Structural changes of samples of S as a result of partial extraction of Na with acid treatment were explored. Strongly and weakly hydrogen-bonded hydroxyls with non-uniform distribution of hydrogen bond strengths were revealed. Decreasing of Na concentration is followed with partial polymerization of the glass framework and hydroxylation of the surface. Deuterium exchange and the thermal dependence of the dehydration of silica–alumina fiberglass (CCB18 and CCB20) were monitored using ¹H and ²⁹Si MAS technique in order to classify surface hydroxyl groups. These samples were prepared by full extraction of Na from alumina–sodium–silicate fiberglass materials similar to S. At least four types of hydroxyl groups were revealed for both samples: internal isolated accessible silanols (2.65 ppm), internal silanols (6.7 ppm and 3.7 ppm) with low accessibility, molecular water displaying broad resonance at 5 ± 1 ppm. Spin–spin relaxation times T₂ were measured for these silanol groups using a standard echo sequence. The full content of water and silanol groups were determined.