The condensation of benzaldehyde and acetone to aldol has been investigated in the liquid phase at 273 K on hydrotalcites transformed into basic solids by activation following different procedures. No activity was observed on the pure hydrotalcite (carbonated) or on the solid just decarbonated at 723 K. The activity went through a maximum after calcination at 723 K, followed by rehydration by water vapor at room temperature. The calorimetric determination of the adsorption of carbon dioxide demonstrated a lower basicity on the rehydrated surface, compared to the mixed oxide. Traces of chlorine retained in the synthesis were detrimental for catalytic properties. An aldol yield higher than 85 mol% was obtained after optimum activation. The higher activity of the rehydrated sample suggests that aldolisation in the heterogeneous phase is specifically catalyzed by hydroxide ions.