The influence of the addition of vulcanizing additives on the molecular dynamics of unvulcanized natural rubber (NR) has been studied by broadband dielectric spectroscopy. Results reveal a slowdown of the segmental dynamics of NR, restricting the motion of the chains tightly bounded to the additive surfaces. In general, when zinc oxide is present a second dynamic process is detected, ascribed to strong interfacial interactions between the zinc ionic clusters and the NR polymer segments. The study of such interactions prior to vulcanization is a very useful strategy to control the vulcanization process, maximizing its benefits and, hence, improving the final products. Here, we demonstrate that broadband dielectric spectroscopy is a good experimental alternative in order to obtain a deeper insight into the vulcanization mechanisms. Furthermore, our results support one previously proposed sulfur vulcanization mechanism, in which molecules of accelerators, sulfur, and fatty acids are adsorbed on the zinc oxide surface.