The present study employs thermogravimetric analysis to explore the thermal stability of malonic acid (MA) dissolved in several polymer matrices that include poly(vinylpyrrolidone) (PVP), poly(methyl methacrylate) (PMMA), and poly(vinyl acetate) (PVAc). It is found that PVP/MA system decomposes at markedly lower temperature than PMMA/MA and PVAc/MA systems. Isoconversional kinetic analysis indicates that the Arrhenius parameters for decomposition of PMMA/MA and PVAc/MA systems are virtually the same as for pure MA. For the PVP/MA system, the Arrhenius parameters differ significantly from the respective parameters for pure MA. The diminished thermal stability of PVP/MA system is linked to the rate constant being considerably larger than in all other systems under study. The effect is explained by a catalytic action of an amino group of PVP, which is similar to the known effect of catalysis of decarboxylation reactions by tertiary amines. The results obtained have important implications for the widespread usage of PVP as a polymer matrix.