Raw natural rubber samples of different Mooney viscosity values and inherent molar mass distributions (MMD) were used to characterize changes occurring during high-temperature mastication and to identify the structural components conditioning mastication-induced degradation. The macrogel degraded rapidly to essentially microgel, the latter of which was also continuously degraded, and at times completely. Mastication provoked decreases in the average molar mass of polyisoprene chains and changes in their MMD. The MMDs evolved differently as mastication progressed, differences that could be linked to the inherent MMD of each rubber. The non-viscosity stabilized grade rubber (TSR 10) of monomodal MMD was progressively degraded by mastication, whereas rubber of the same grade but with a bimodal MMD was degraded much slowly and even stagnated on prolonged mastication. The stage at which stagnation occurred was characterized by an offset of the balance between long and short polyisoprene chains due to increases in the quantity of short chains, which facilitate the sliding of the longer ones.