The thermal evolution, from room temperature up to 300 degrees C, of layered double hydroxides (LDHs) with (Ni, Co) or (Ni, Fe) based slabs and containing intercalated carbonate anions, was studied. A new lamellar phase, exhibiting a 6.5-6.6 A interslab distance, was observed in the 200-260 degrees C temperature range. The infrared study and the chemical analysis results allowed us to give prominence to a monografting of carbonate anions to the slabs. The thermogravimetric studies seem to indicate that intercalated water is first lost, followed by a monografting of carbonate anions. A stay in water of the 200 degrees C thermally treated phase leads to a reintercalation of water molecules. The interslab distance indeed reincreases up to the value of the pristine material, reinforcing the hypothesis of the monografing.