Infrared spectra of acetophenone adsorbed on mordenite and acid-leached mordenite are reported. The effects of dealumination were characterized by pyridine adsorption and 29 Si and 27 Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Adsorption of pyridine gave bands at 1634 and 1545 cm Ψ1 due to vibrations of pyridinium ions and a band at 1621 cm Ψ1 characteristic of pyridine ligated to Lewis acidic Al 3Ψ sites. 27 Al NMR gave peaks at 55 ppm due to tetrahedral framework Al and at 0 ppm due to octahedral extraframework Al. 29 Si NMR gave peaks at Ψ106 and Ψ112 ppm due to Si(1Al) and Si(OAl) species, respectively. For acetophenone on mordenite, spectra contained (CAO) bands at 1685, 1671, 1650, and 1630 cm Ψ1 due to physisorbed acetophenone containing unperturbed CAO groups, acetophenone hydrogen bonded with external and internal silanol groups, acetophenone hydrogen bonded to BrΓΈnsted acidic Si(O-H)Al groups, and acetophenone ligated to Lewis acidic Al 3Ψ ions, respectively. Dealumination converted 56% of the Si(1Al) species to Si(OAl), reduced the concentrations of both framework and extraframework Al, reduced the concentration of Si(OH)Al groups, and greatly reduced the population of Lewis acidic sites, but generated internal silanol groups which formed hydrogen bonds with acetophenone molecules. Residual Al 3Ψ sites and Si(O-H)Al groups after dealumination could interact with adsorbed pyridine molecules but for steric reasons were unable to form adsorption complexes with acetophenone. Evidence for the protonation of acetophenone was inconclusive.