The acid-base properties of supported gallium oxide catalysts have been investigated by microcalorimetry and MR spectroscopy of pyridine (acidity) and CO2 (basicity) adsorption. Pyridine adsorption microcalorimetry has shown that loading supports such as gamma -Al2O3, TiO2, and ZrO2 with gallium oxide in a surface concentration close to the theoretical monolayer, resulted in a decrease of the surface acidity of the catalysts compared to that of the supports, while, in the case of SiO2, new Lewis acid sites were created. The number of acid sites for the supported and bulk gallium oxide catalysts was in the following order: Ga2O3/SiO2 < Ga2O3/ZrO2 < Ga2O3/TiO2 approximate to Ga2O3 < Ga2O3/Al2O3. This order is related to the degree of dispersion of gallium oxide on the surface of each support and to the interaction between guest and host oxides. Two types of Lewis acid sites were identified by pyridine adsorption, corresponding to the support and the supported gallium oxide, respectively. The catalysts displayed very weak basicity, except for the samples supported on zirconia and alumina.