The surface＇s acidity of amorphous and mesoporous MCM-41-type silica and silica-aluminas has been investigated by using IR spectroscopy of the surface OHs and of adsorbed pivalonitrile, and catalytic isomerization of 1-butene. IR experiments allowed also characterizing the external sites of ferrierite. In contradiction to part of the recent literature, the conclusion is that the Bronsted acidic OHs associated to Al insertion into the silica framework are terminal on the surfaces of both amorphous and mesoporous samples and also on the external surface of ferrierite. These sites participate to the generation of the O-H stretching band near 3745 cm(-1). These surfaces also carry Lewis acid sites arising from Al cations in a tetrahedral-like unsaturated coordination sphere. It is concluded that bridging Si-OH-Al ions, giving rise to strong Bronsted acidity of protonic zeolites, are only forced to be formed into their internal cavities, but cannot occur on "flat" or sufficiently "open" surfaces.