Microcalorimetric and infrared spectroscopy studies of ammonia and carbon dioxide adsorption were carried out to investigate the number, strength, and type of acid/base sites of europium oxide supported on gamma-Al2O3 and SiO2. X-ray diffraction, luminescence spectroscopy, and Mossbauer spectroscopy were used to characterize the structure, bonding, and valence of the europium atoms. Europium oxide neutralizes acid sites on gamma-Al2O3, lowering the initial heat of ammonia adsorption from 155 to 120 kJ/mol. Both Lewis and Bronsted acid sites exist on Eu2O3/Al2O3 samples, with Lewis acid sites in dominance. Europium oxide increases the number and strength of base sites on gamma-Al2O3, with the initial heat of carbon dioxide adsorption increasing from 145 to 160 kJ/mol. Surface hydroxyl groups and oxygen anions are responsible for base sites on the Eu2O3/ Al2O3 samples. The addition of europium oxide to SiO2 increases the number and strength of acid sites, with the initial heat for ammonia adsorption increasing from 85 to 125 kJ/mol. Europium oxide on silica shows few basic sites. Luminescence spectra show that EU(3+) ions OCCUPY a range of dissimilar bonding sites on both supports, particularly at low concentrations. At the higher concentrations, EU(3+) ions occupy more well-defined sites, having a weaker interaction with the support. The acid/base properties of these supported europium oxides can be related to the electronegativities of these samples, with higher electronegativity enhancing acidic properties and suppressing basic properties.