This study reexamines an early literature report of an unusual phenomenon of a dramatic difference in the activation energy for the thermal decomposition of malonic (propanedioic) acid in the liquid and solid states. The study has been carried out via thermogravimetric analysis (TGA) to probe possible differences between the kinetics in three condensed phases: solid, liquid, and supercooled liquid. An advanced isoconversional method has been applied to determine the activation energy, preexponential factor, and reaction model. The activation energy as well as preexponential factor for the decomposition processes in all three condensed phases have been determined to be practically the same: E = 110 +/- 10 kJ mol(-1) and log(A/min(-1)) = 13 +/- 1. In all phases, the reaction kinetics has been found to follow the reaction order model. The reaction order values have been similar for the liquid and supercooled liquid phases but markedly smaller for the solid-phase decomposition. The results obtained have not confirmed the phenomenon reported in the early literature. A practically important conclusion of the present study is that the liquid-phase kinetic data can be used to obtain a reasonable estimate for the thermal stability of this compound in the solid phase.