Silanol groups existing on the outer surfaces of zeolites, whose OH stretching band appears at the same frequency as that of silanol groups on amorphous silica, are recognized as neutral or very weakly acidic. No significant difference in acidity between silanol groups on zeolites and those on amorphous silica has yet been found by studying the adsorption of probe molecules. However, it was found by a "reaction probe IR study" that silanol groups involved in highly crystallized structures of zeolites (ferrierite and ZSM-5) donated their protons to olefins (2-methylpropene, 2,4,4-trimethyl-1-pentene, and 2,4,4-trimethyl-2-pentene) at temperatures lower than 220 K, in contrast to those on amorphous silica. The products formed from reaction of those olefins with silanol groups were identical to 2,4,4- trimethyl-2-pentoxy groups ((CH3)(3)C-CH2-C(CH3)(2)-O-Si). These results suggest a difference in acidity between silanol groups on zeolites and those on amorphous silica.