The electron paramagnetic resonance (EPR) spectra of Mn2+ have been used to investigate the structure of silica xerogels prepared from tetraethylorthosilicate. The measurements were made at ∼ 9.8 GHz. The probe was introduced during preparation of a sol and the EPR spectra of subsequent xerogels were measured following calcination at various temperatures (120°C ≤ TT ≤, 1000°C). The spectra after adsorption of NH3 or H2O were measured. Adsorption of NH3 showed a quasi non-reversible effect whereas there was a modification of the spectra which depended on the water vapor pressure. Adsorption and desorption of water were ascribed to surface effects and simulation of the spectra showed that two different sites are involved. An EPR method of determination of pore diameter is proposed.