The decomposition under thermal treatments of an hydrotalcite sample prepared from Mg and Al chlorides, thus containing Cl- and CO32- as compensating anions, and of the same sample totally exchanged by CO32-, was followed by several characterization techniques (in situ XRD, Al-27 NMR, TG, chemical analysis). A similar structural evolution is found, whatever the compensating anion, with a progressive collapse of the interlayer space up to 573 K and an expansion of the lattice alpha parameter related to an extraction from the lattice of Al3+, whose ionic radius is slightly lower than that of Mg2+. At higher calcination temperatures, a Mg(Al)O phase is formed and a small amount of tetrahedrally coordinated Al and a line at d = 0.254 nm in the XRD pattern are detected. On the other hand, more Al is extracted from the chlorinated than from the carbonated samples, either noncalcined or calcined at increasing temperatures, when treated with an alkaline solution (NaOH). The aluminum is removed at the reverse of chlorine. Therefore Al is preferably removed from the weaker basic sites not blocked by Cl-. This obviously shows that the restoration of a calcined LDH could not be totally reversible. Nevertheless the lamellar structure is restored when dispersed in water, with a different composition than that of the starting material. The adsorption of pyrrole studied by IR spectroscopy reveals an interaction of this probe mainly in its undissociated form with the OH groups. Rather different species are formed depending on whether the sample contains Cl- or not. From these species, an increase of the basicity is qualitatively inferred following the removal of Cl-.