The thermal stability of N,N'-dipentamethylenethiuram disulfide (CPTD) and tetramethylthiuram disulfide (TMTD) with respect to thiourea formation was compared. CPTD is stable at 120 degrees C but decomposes to N,N'-dipentamethylenethiourea (CPTU) at vulcanization temperatures (140 degrees C), while TMTD is stable to 190 degrees C. These differences do not reside in residual impurities in the compounds. Amines, which catalyze thiourea formation, originate on decomposition of alkyldithiocarbamic acids, and these form more readily in CPTD than in TMTD since the abstraction by thiuram sulfenyl radicals of hydrogen from methylene in CPTD is easier than from methyl in TMTD. ZnO and the zinc-accelerator complex, zinc bis(pentamethylenedithiocarbamate), inhibit degradation by trapping the alkyldithiocarbamic acid before it can decompose to piperidene, that is, stabilization is achieved when the precursor to the amine, rather than the amine itself, is removed from the system.