FTIR spectroscopy provides evidence that carbon monoxide interacts with surface silanol groups of silica and silicalite to form both SiOH...CO and SiOH...OC hydrogen-bonded complexes. The C-bonded adduct shows a characteristic IR absorption band which is blue-shifted as compared to free CO (2143 cm(-1)), while the O-bonded adduct is characterized by a red-shifted band. Variable temperature IR spectroscopy has shown that these two hydrogen-bonded adducts are in a temperature-dependent equilibrium which involves an enthalpy change of about 2-3 U mol(-1). Spectroscopic data are supported by ab initio cluster model calculations of different complexity levels. (C) 2003 Elsevier Science B.V. All rights reserved.