Thermogravimetry in combination with isoconversional kinetic analysis is utilized to probe the effect of poly(vinylpyrrolidone) (PVP) on the decomposition of malonic acid (MA) in PVP matrices. The systems under study contain 4, 2, and 1 mol of PVP per 1 mol of MA. Although all systems demonstrate acceleration relative to neat MA, the strongest effect per mole of added PVP is observed in the 1:1 system. In the 4:1 system the acceleration effect weakens with the progress of decomposition and ultimately turns into deceleration. Kinetic analysis suggests that in the 1:1 and 2:1 systems, the decomposition rate at the advanced stages of the process is limited by vaporization of acetic acid. The unusual behavior of the 4:1 system at the advanced stages is proposed to be due to the decomposition rate being limited by a reversible step of dissolution of acetic acid in an excess of PVP.