The layered double hydroxides (LDHs) of Mg with Ga and In decompose completely on heating to 500 degrees C to yield poorly ordered oxide residues. In the Mg-Ga system, the oxide residue has a rock-salt structure with Ga incorporated in the MgO matrix. In the Mg-In system, the oxide residue is X-ray amorphous. These oxide residues revert back to the original LDH either on standing in a water-saturated atmosphere or on hydrothermal treatment in a Na2CO3 solution. In contrast, the LDHs of Co with Ga and In yield the thermodynamically stable spinel oxides and the decomposition is irreversible. These results have implications for the synthesis of 'oxide' catalysts by thermal decomposition of the LDHs. (c) 2005 Elsevier Ltd. All rights reserved.