A combination of high resolution thermogravimetric analysis coupled to a gas evolution mass spectrometer has been used to study the thermal decomposition of synthetic hydrotalcites reevesite (Ni₆Fe₂(CO₃)(OH)₁₆·4H₂O) and pyroaurite (Mg₆Fe₂(SO₄,CO₃)(OH)₁₆·4H₂O) and the cationic mixtures of the two minerals. XRD patterns show the hydrotalcites are layered structures with interspacing distances of around 8.0. Å. A linear relationship is observed for the d(001) spacing as Ni is replaced by Mg in the progression from reevesite to pyroaurite. The significance of this result means the interlayer spacing in these hydrotalcites is cation dependent. High resolution thermal analysis shows the decomposition takes place in 3 steps. A mechanism for the thermal decomposition is proposed based upon the loss of water, hydroxyl units, oxygen and carbon dioxide.