The method of differential thermal analysis has been applied to the study of thermal effects accompanying exothermic vulcanization reaction in hard rubber compounds accelerated with common rubber accelerators. Exothermic reaction was found to start at about 1% sulfur, in the case of CBS-accelerated compound which increases linearly up to 32% and then decreases, probably due to predominance of a dehydrogenation reaction. 68/32 Rubber-sulfur compounds containing increasing amounts of combined sulfur evolve decreasing quantity of heat and a plot of ΔH vs. combined sulfur of the precured compounds shows three stages of reaction. Accelerators were found not to affect the heat of reaction of the compounds to a great extent except in the special cases of those containing MBT and ZDC. The beneficial effect of certain accelerators is to be traced in the lowering of initiation temperatures and a decrease of slope values whereby the reaction becomes more regulated and spread out. Increase of concentration of accelerators (CBS and TMT) was found to have little effect.