The interactions between the acetone carbonyl and the carbonyl and double bond of mesityl oxide and the Bronsted and Lewis acid sites of acid catalysts have been studied by Fourier Transform infrared (FTIR) spectroscopy. The acetone carbonyl interactions with bridging acidic OH and with bridging weakly acidic OH can be distinguished from one another semiquantitatively. This was substantiated by a quasi one-to-one relation between the number of bridging OH in a cluster Si-nAl containing only one Al and the number of strongly hydrogen-bonded acetone. The distinction between Bronsted and Lewis sites is not easy with acetone, while it is with mesityl oxide. The system acetone-mesityl oxide adsorbed an acid catalysts is rather unique, since the reagent and the reaction product of the condensation reaction are qualitatively and quantitatively detectable by FTIR spectroscopy. The quantitative determination of the coverage of the acid sites by the reagent and the reaction product was a prerequisite to the study of the reaction presented elsewhere.