The thermal dehydration and decomposition of alkaline earth metal malonate hydrates and their anhydrides have been studied using TG-DTA, IR spectroscopy, X-ray diffractometry and evolved gas analysis in atmospheres of N-2, CO2, O-2 and air. The initial temperatures of the dehydration t(i) and decomposition T-i which can measure the relative stabilities of the hydrated and anhydrous malonates are given in various atmospheres. The t(i) values on the TG curves were shown to be a measure of the relative strengths of the M(2+)-OH2 bond. The plot of the T-i values against Kahwa’s parameter for alkaline earth metal malonate anhydrides, except Be, yielded a straight line. Analysis of the first dehydration and decomposition steps indicated that the R2 and A3 mechanisms for the dehydration and decomposition processes give the best fit of the results, respectively.