The IR spectroscopic methods for the characterization of the surface acidity of metal oxide catalysts are briefly reviewed. The use of different basic probe molecules whose IR spectra are sensitive to protonation and/or to the strength of Lewis acid-base interaction is described. The results obtained for the characterization of the Lewis acid strength of more than 30 binary and ternary mixed oxides are interpreted on the basis of the different polarizing powers of the involved cations. The characterization of the Bronsted acidity of several oxides and zeolites made by using the "basic strength method", the "hydrogen bonding method" and the "olefin polymerization method" is also described.