The thermal behaviour of synthetic hydrotalcites, Mg6Al2(OH)(16)CO3. nH(2)O, Co6Al2(OH)(16)CO3. nH(2)O and Ni6Al2(OH)(16)- CO3. nH(2)O, has been studied by thermogravimetric analysis (TGA/DTA) and infrared emission spectroscopy (IES), The Mg-hydrotalcite shows a weight loss up to 70 degrees C due to physically adsorbed/interlayer water. Between 70 degrees C and 230 degrees C weight is lost due to decarbonisation (DTA endotherm 222 degrees C). The dehydroxylation basically takes place between 230 degrees C and 420 degrees C (DTA endotherm 400 degrees C). The different steps are more overlapping for the Co-hydrotalcite. In contrast, the DTA of Ni-hydrotalcite shows a complex of endothermic maxima between 73 degrees C and 207 degrees C followed by a strong endotherm at 323 degrees C. The Mg-hydrotalcite IFS show major changes around 350-400 degrees C indicating the end of the dehydroxylation. In this temperature range 'Al'-OH bands disappear and some new bands are observed indicating the formation of spinel (MgAl2O4) together with MgO, which is another product formed after dehydroxylation. Similar results are obtained for the Co-hydrotalcite, which shows the formation of CoAl2O4 spinel and/or cobalt-oxide. All bands in the takovite IES at 200 degrees C disappear at 300-350 degrees C, while new bands are formed suggesting the formation of a spinel-like phase rather than of Al2O3. So, IES supports the reaction of Ni-hydrotalcite to a NiAl2O4 spinel phase+NiO and not Al2O3+NiO.