The interaction of HCl with an eta-alumina catalyst has been investigated by a combination of diffuse reflectance infrared spectroscopy, temperature-pro.-rammed desorption and inelastic neutron scattering (INS) spectroscopy. Infrared spectra provide evidence for dissociative adsorption of HCl and for a process in which hydroxyl groups terminally bound to Al are replaced by chlorine. Temperature-programmed desorption experiments show HCl to desorb over the temperature range 350-> 970 K, indicating dissociative HCl adsorption to occur on a wide range of active sites. INS experiments show the residual alumina hydroxyl groups to exhibit an out-of-plane deformation feature, gamma(OH), at ca. 200 cm(-1). while the in-plane deformation mode, 8(OH), is seen at ca. 1000 cm(-1). The formation of new surface hydroxyl groups via the adsorption of hydrogen chloride yields a 8(OH) feature that can be resolved into two bands at 990 and 1050 cm(-1). Hydrogen bonding within the alumina/HCl system is responsible for the observation of an Evans transmission window in the infrared spectrum, that occurs via a Fermi resonance interaction between (i) the v(OH) mode of hydrogen bonded hydroxyl groups and chemisorbed water with (ii) the overtone of the 8(OH) mode of surface hydroxyl groups. The INS technique is able to discriminate among different hydroxyl group bonding geometries on the basis of the local symmetry of the active sites. (c) 2006 Elsevier B.V. All rights reserved.