Magnesium-aluminum hydrotalcite compounds have been synthesized without possible contamination by alkali cations. Pure hydrotalcite compounds with Mg/Al ratios from 3 to 12 were synthesized without the formation of separate phases. These materials were converted into magnesium-aluminum mixed oxides upon calcination at 773 K, showing the X-ray diffraction pattern of MgO. Microcalorimetric and infrared spectroscopic measurements of NH3 and CO2 adsorption were used to characterized the number, strength, and nature of acid/base sites on the surface of the calcined hydrotalcites. These materials dod not possess acid sites and base sites with differential heats higher than 120 and 150 kJ/mol, respectively. In contrast, gamma-Al2O3 possesses acid sites with differential heats from about 60 to 160 kJ/Mol, and MgO has base sites with differential heats as high as 170 kJ/mol. Infrared spectroscopy revealed that the acid sites on calcined hydrotalcites are mainly Lewis acid sites, while the base sites are oxygen anions. Calcination of hydrotalcites can be used to prepare Mg-Al-O mixed oxides with high surface area and moderate acidity and basicity.