In a recent priority communication [M. Digne et al., J. Catal. 211 (2002) 11, we proposed the first ab initio constructed models of gamma-alumina surfaces. Using the same density-functional approach, we investigate in further detail the acid-basic properties of the three relevant gamma-alumina (100), (110), and (111) surfaces, taking into account the temperature-dependent hydroxyl surface coverages. The simulations, compared fruitfully with many available experimental data, enable us to solve the challenging assignment of the OH-stretching frequencies, as obtained from infrared (IR) spectroscopy. The precise nature of the acid surface sites (concentrations and strengths) is also determined. The acid strengths are quantified by simulating the adsorption of relevant probe molecules such as CO and pyridine in correlation with surface electronic properties. These results seriously challenge the historical model of a defective spinel for gamma-alumina and establish the basis for a more rigorous description of the acid-basic properties of gamma-alumina. (C) 2004 Elsevier Inc. All rights reserved.