The thermal evolution, from room temperature up to 300 degreesC, of layered double hydroxides (LDHs) with (Ni, Cc) or (Ni, Fe) based slabs and containing intercalated carbonate anions, was studied, A new lamellar phase, exhibiting a 6.5-6,6 Angstrom interslab distance, was observed in the 200-260 degreesC 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 degreesC 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.