Water adsorbed on pyrogenic silica modified by phosphorus chlorides, which were then hydrolyzed, was studied by H-1 NMR, dielectric, and IR spectroscopies and quantum chemical ab initio and semiempirical AM1 and MNDO/H methods. Hydrophosphinic groups (HPA) bound to pyrogenic silica surfaces are stable at the adsorbed water content C-w less than or equal to 12 wt %, which corresponds to water adsorption at standard conditions. Increase of the adsorbed water content leads to hydrolysis of the =Si-O-P bonds via nucleophilic substitution S(N)i(Si) mechanism. Upon heating, the adsorbed phosphinic acid can cleave the =Si-O-Si= bridges and form new sites responsible for changes of structure of the adsorbed water clusters and chemical shift values for the H-1 NMR spectra. Great clusters of the adsorbed water molecules, which form at C-w > 12 wt %, are localized near ionized bound hydrophosphinic groups or adsorbed acid molecules. For air/H2O/HPA/SiO2, the adsorption energy of water decreases if the water content increases. The dielectric and 1H NMR spectra of H2O/HPA/SiO2 have significant distinctions in comparison with those for the parent pyrogenic silica, and these differences depend not only on the adsorbed water content but also on reaction temperature and other treatment parameters.