A study on the catalytic properties of properly activated hydrotalcite (HT) with special attention to the nature and amount of active sites present in this solid base catalyst has been undertaken. Only a small fraction (5%) of the available basic sites in the rehydrated calcined HT is active in liquid-phase aldol condensations. These sites exhibit high catalytic activity and are most likely localized at the edges of the MT-platelets. Besides a high activity, these modified HTs also show a high selectivity. No further condensation products other than diacetone alcohol (DAA) in the acetone self-condensation could be observed. Initial results with the citral-acetone condensation show that even at 273 K this reaction is catalyzed by modified HTs with a conversion of 65% and a selectivity of 90%, when the citral concentration is not too high (similar to 1wt.%). At higher citral concentrations, no reaction is observed indicating a negative order in citral concentration.