The present study is dedicated to the characterization (identification, heats of adsorption, and coverages) of the adsorbed species formed by the adsorption and coadsorption of NH3 and H2O on two SiO2 solids. Adsorption equilibrium infrared spectroscopy allowed us (a) to show that NH3 and H2O are mostly adsorbed on free SiOH groups via H bonds and (b) to determine their individual heats of adsorption: 53 and 49 kJ/mol, whatever be their coverages (Langmuir adsorption model), for NH3ads and H2Oads, respectively. These values consistent with the microcalorimetry literature data explain that their coverages are decreased upon NH3-H2O coadsorption, considering a competitive Langmuir model. However, the temperature-programmed adsorption equilibrium procedure achieved from MS data indicated that a minor amount of other NH3 species (not detected using Fourier-transform infrared) is more strongly adsorbed and that hydrolysis of SiOSi siloxane by H2O could occur in parallel. NH3-H2O coadsorption leads to the formation of NH4+ species, which involves H2O adsorbed species. Both NH3 and H2O are not adsorbed above 450 K, which means that the SiO2 contribution to the characterization of the acidity of metal oxide catalysts supported on SiO2 using NH3 as the probe molecule in the presence of H2O is negligible above this temperature.