Synthetic manganeous Layered Double Hydroxides of the Hydrotalcite- and Shigaite-type were prepared by the coprecipitation method. The thermal behaviour of two compounds, [Mn6Al3(OH)(18)] [(SO4)(2)Na(.)12H(2)O] and [Mn6Al3(OH)(18)] [Cl(.)4H(2)O] was investigated by several experimental techniques. Thermal analysis and in-situ X-ray diffractometry show several dehydration stages of the synthesis products in the temperature range of 25 < T < 170degreesC followed by decomposition at temperatures above 200degreesC.