Surface silanol groups of silica, which never revealed any Brønsted acidity, are shown to donate protons to adsorbed basic molecules, such as ammonia, pyridine or 2,6-dimethylpyridine, after addition of acids such as SO₂ or NO₂. The latter, when coadsorbed with bases, interact with the oxygen atoms of silanols leading to OH acidity increase and to protonation. Bases, in turn, enhance chemisorption of SO₂ or NO₂, and strongly held coadsorption products are formed as a result. The proposed mechanism of induced Brønsted acidity could account for the promoting effect of acidic gases in reactions catalysed by metal oxides.