This paper presents the results of thermoanalytical studies (TG/DTA) to examine the influence of additives (MgO, Al2O3 or SiO2) on the thermal stability of SrCO3 and BaCO3. Since any destabilization is expected to be maximised when the carbonates and the additives are in intimate contact, viz., solid solution, the possibility of the formation of solid solution and other stoichiometric compounds has been explored and the phases identified by X-ray diffraction (XRD). Thermogravimetry data show that the incorporation of MgO in SrCO3 and BaCO3 lowers the carbonate decomposition temperatures by as much as about 150-degrees-C. Destabilization is also observed using alumina or silica, although the extent to which this occurs is moderate compared with MgO as an additive. Moreover, SiO2 interacts strongly and irreversibly forming Ba2SiO4, BaSiO3 or Sr2SiO4 and SrSiO3. Al2O3 also interacts, albeit mildly, forming BaAl2O4 or SrAl2O4. A solid solution of the type (MgBa)(CO3)2 is the only phase detected by XRD in the coprecipitated carbonates containing magnesium. Carbonate stability depends on the environment. Whilst a carbon dioxide environment suppresses decomposition, hydrogen enhances decomposition through provision of another reaction channel.