Adsorption of binary mixtures of gases with acidic and basic properties on silica surface has been studied by FTIR spectroscopy at low temperatures. After introducing acidic molecules to silica sample with preadsorbed base, additional shifts of the band of silanol groups already perturbed by bases occur that are interpreted as caused by interaction of the acid with the oxygen atom of silanol group, leading to the H-bond strengthening. Comparison of the effects produced by different acidic molecules enables an estimate of their relative electron-accepting (acidic) properties to be made, including the acidity of ozone that is not easy to estimate by other methods. For strong bases, such as ammonia, pyridine or 2,6-dimethylpyridine, addition of SO₂ or NO₂ enhances the acidity of silanol groups to such extent that proton transfer occurs, indicating the appearance of Brønsted acidity induced by coadsorbed acids.