Evidence about the remarkable acidic nature of molecular water adsorbed at the surface of mesoporous MCM-41 silicoaluminas (MSA) with a ratio of Si/Al = 25 is reported, coming from both infrared spectroscopy and ab initio calculations. By using CO as a probe, OH stretching and HOH bending modes of water adsorbed on coordinatively unsaturated Al ions (Lewis sites) at the surface of MSA have been detected for the first time. CO adsorption causes OH stretching frequencies to suffer a bathochromic shift of about -200/-250 cm(-1), whereas the HOH bending frequency undergoes a hypsochromic shift by about +10/+20 cm(-1). B3LYP/6-31+G(d,p) calculations on model clusters designed to mimic both Brønsted and Lewis sites validate the assignments, showing that water adsorbed on the surface Lewis site, when interacting with CO, shows an acidity comparable to that of a classical zeolitic Brønsted site.