Abstract The hplc studies with all their limitations have been employed to obtain information regarding the vulcanization of rubber accelerated by CTOS and MBTS in the early part of the reaction. It is noticed that even in the initial part of the induction period (the scorch safety, t2, of the recipe is 10.5 min), CTOS and MBTS react so rapidly with each other and also with rubber that at the end of 10 min we could detect only MBT, while the concentration of the other components formed in the reaction—namely, CTOS, OBTS, CPTD, BPTD, PMTU, CPTM, BPTM, etc.—decreased to an extremely low level. We noticed that under the experimental conditions, OBTS and CPTU have the same retention time. This naturally obscures the path for understanding the individual effect of these accelerators. The sharp disappearance of OBTS, associated with the abundant formation of MBT and nonavailability of CPTU (from CPTD, Figure 11b), gives testimony to the fact that the unsymmetrical thiourea (PMTU) rather than the symmetrical one (CPTU) is solely formed in the vulcanizates under discussion. The accelerating as well as retarding effect of thiourea has been reported by Dućhac^ek in the vulcanization of NR. Substituted thiourea, namely, bis(oxydiethylene) thiourea, has also been shown to influence the vulcanization of SBR in the presence of a mixture of accelerators formed by the early reaction of OTOS and OBTS. The influence of PMTU, however, remains to be investigated, and studies in this line are being pursued. Also, we could not isolate the effect of MDB from that of CPTD, since both of them have the same retention time. From our experience, we know that in comparison with CPTD, the proportion of MDB formed under the experimental conditions is very low and, hence, it is believed that CPTD plays the major role. It has also been observed that BPTD rapidly transforms into BPTM which, as noticed by us, also influences vulcanization. It is concluded that in the binary system containing CTOS and MBTS, the intermediate accelerators, OBTS, CPTD, and BPTD play a vital role, and their growth and decay, either in the presence or absence of the ZDC, control the fate of the vulcanization reaction.