The rate of formation of tetramethylthiuram polysulfides (TMTP), that play an important role in vulcanization, was studied. After a short induction period (< 30 s), tetramethylthiuram disulfide (TMTD) and TMTD-sulfur mixes, heated to 130-150-degrees-C in the absence of rubber, rapidly form a series of TMTPs. The concentrations of TMTPs of lower sulfur rank increase most rapidly, indicating that sulfur atoms are added to the accelerator sequentially. The incorporation of sulfur molecules to give TMTPs, which subsequently desulfurate, does not occur. Equilibrium concentrations of the various TMTPs are achieved in about 2 min. Little tetramethylthiourea is formed below 200-degrees-C. Tetramethylthiuram monosulfide (TMTM) is stable, but TMTM-sulfur mixes form TMTPs. A mechanism is proposed to account for the large amount of TMTM formed on heating TMTD in the absence of sulfur and the correspondingly higher TMTP concentrations in the presence of sulfur.