The IR spectroscopy of HCl adsorbed on silica and on silicalite and of HCl-B (B = (CH3)(2)O, (CH3CH2)(2)O, THF) adducts formed in silicalite channels at similar to 210 K is described in detail. The whole subject is divided in three parts. The first part concerns the IR study of the HCl interaction with silica and silicalite. The formation of weak H-bonded adducts between silanols and HCl molecules has been observed; the number of HCl molecules in the adducts increases with the HCl dosage. The HCl molecules adsorbed on silicalite behave as partially hindered rotators. The experimental results are supported by ab initio and molecular dynamics calculations. In the second part, the TR study of HCl-B complexes (B = (CH3)(2)O, (CH3CH2)(2)O, THF, and benzene) either adsorbed on silica or entrapped in silicalite channels is described. In the third part, the results obtained for HCl-B adducts in silicalite are compared with those obtained in cryogenic matrixes and in solution. It is inferred that the acid strength of HCl is appreciably influenced by the presence of the siliceous framework. This result allows discussion of the effect of long-distance interactions (framework effect) on the acid strength of structural Bronsted sites of zeolites.