Surface silanol groups of silica, which never revealed any Bronsted 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 Bronsted acidity could account for the promoting effect of acidic gases in reactions catalysed by metal oxides.