The isomerization of beta-isophorone to alpha-isophorone has been investigated as a model reaction characteristic of basic sites. A zero order is observed at 308 K on many solid bases, thus allowing a measure of the total number of active sites. On MgAl mixed oxides obtained by decarbonation of hydrotalcites, this number corresponds to the number of sites adsorbing CO2, and a linear correlation is observed between the initial rates of isomerization of isophorone and those of Claisen-Schmidt aldolic condensation, thus showing that both reactions are catalyzed by basic sites. By rehydration the mixed oxides are converted to meixneritelike materials, with an increase of activity for isomerization, as for aldolization. The rate of isomerization has been used to measure the number of sites on a series of solid bases, the basic strength of which was estimated by the temperature of decarbonation. With this criterion, Ba hexa-aluminate decarbonated below 550 K appears to be a weaker base than hydrotalcite and La2O3/MgO decarbonated above 650 K and KF/alphaAl(2)O(3) decarbonated above 850 K. The activities of these solids for isophorone isomerization are, however, proportional to the number of sites determined by desorption Of CO2.