The purpose of this investigation was to determine the nature of radicals present on Natural rubber and poly isoprene system. The qualification and quantification of radicals was made by using Electron Paramagnetic Resonance spectroscopy (EPR). The mechanism of vulcanization has been studied in order to understand the formation of organic radicals. This could give us indications as to the nature of the sulfur chains formed during the process. Thanks to the in situ EPR we were able to highlight the formation of radicals S3•− which quickly transforms into a stable carbon radical. We can therefore assume in this radical mechanism that the length of sulfur chains observed is 3 sulfur. Sulfur containing more than 3 atoms would be created following an ionic pathway. These results have led us, once the radicals formed have been stabilized, to study the phenomena of thermal aging by monitoring the nature and quantity of radicals formed over longer times. These results allowed us to show the existence of oxygen vacancies which are transient species linked to the presence of ZnO used as an activator of vulcanization.