A combination of high resolution thermogravimetric analysis coupled to a gas evolution mass spectrometer combined with infrared emission spectroscopy has been used to study the thermal decomposition of synthetic hydrotalcites honessite (Ni6Fe2(SO4)(OH)(16)(.)4H(2)O) and mountkeithite (Mg6Fe2(SO4)(OH)(16)(.)4H(2)O) and the cationic mixtures of the two minerals. High resolution thermal analysis shows the decomposition takes place in five steps. A mass loss step is observed over the 125-150degreesC temperature range and is attributed to the mass loss due to dehydration. A second mass loss step is observed over the 260-330 degreesC temperature range and is attributed to dehydroxylation. The third mass loss occurs from 350 to 460 degreesC, and is assigned to a loss of oxygen. The fourth mass loss step is ascribed to the loss of sulphate from the hydrotalcite and occurs over the 676-820 degreesC temperature range. A mechanism for the thermal decomposition is proposed based upon the loss of water, hydroxyl units, oxygen and sulphur dioxide. The changes in the chemical structures are readily followed by infrared emission spectroscopy. (C) 2004 Elsevier B.V. All rights reserved.