On the surface of two commercial pyrogenic silicas (Degussa and Cabot), five resonances were identified on the basis of the chemical shift, homonuclear coupling (T-2), and spin-lattice relaxation behavior (T-1). In accordance with previous studies we observed three different types of silanol groups : (i) weakly coupled (long T-2), water inaccessible, isolated "internal" silanols at 1.8 ppm; (ii) weakly coupled, external "free" silanols revealed upon dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between 3 and 7 ppm. The resonance of water, whose position between 2.6 and 4.6 ppm depended on water content, corresponded to two unresolved species of slightly different T-1. By equating this resonance to the weighted average of two distinct populations of water, we were able to distinguish the first layer of strongly hydrogen bound water at 2.7 ppm from liquid-like water at 5 ppm. The first layer is complete for water relative humidity as low as 3.6% and corresponds to a surface coverage of 4.75 H2O/nm(2). If we assumed a cristobalite-based surface structure, this meant a 1:1 ratio between surface hydroxyls and the first layer of physisorbed water. This ratio was the same for the two silicas regardless of surface area.