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 NH[sub 3] and CO[sub 2] adsorption were used to characterize the number, strength, and nature of acid/base sites on the surface of the calcined hydrotalcites. These materials do not possess acid sites and base sites with differential heats higher than 120 kJ/mol, respectively. In contrast, [gamma]-Al[sub 2]O[sub 3] possesses acid sites with differential heats from about 60 to 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 acids sites, while the base sites are oxygen anions. Calcination of hydrotalcites can be used to prepare Mg-Al-O mixed oxides with high surface are and moderate acidity and basicity. 35 refs., 11 figs., 2 tabs.