Thermal decomposition of M-Mg-Al-CO3-NO3 hydrotalcites (where M is Cu2+ Co2+ or Ni2+) with intended M/Mg (0.10/0.6 1) and M/Al (0.10/0.29) ratios prepared under the same conditions was studied using high temperature X-ray diffraction (HT-XRD) and thermogravimetry coupling with mass spectrometry (TGA-DTA-MS). Introduction of transition metals to the brucite-like layers of hydrotalcites changed their thermal behaviour. The decomposition of nitrate or carbonate anions was coupled with oxidation from Ni2+ to Ni2+ and Co2+ to Co3+. Further increase of temperature resulted in reduction of these cations. Reduction from Cu2+ to Cu+ was also observed above 700 degreesC. Stabilisation of carbonate anions by Cu2+ introduced into hydrotalcite matrix was found at temperatures above 600 degreesC. The thermal treatment of hydrotalcite precursors resulted in the formation of mixed oxide and spinel phases, which were stable after cooling to ambient temperature with exception of CuO phase that appeared during cooling down due to segregation effects. Reducibility of the hydrotalcites calcined at different temperatures was determined by temperature-programmed reduction (TPR) experiments. Specific surface areas of hydrotalcites calcined at 600 degreesC ranged from 226 to 196 m(2)/g dropping with increase in the calcination temperatures to values ranging between 138 and 49 m(2)/g depending on transition metal contents.