Vulcanized rubbers have three-dimensional chemical networks, and as a result they do not melt or dissolve. The presence of these networks creates a tremendous problem at the end of a product's life (i.e., recycling). Recently, R. J. Farris et al. have rediscovered a technique coined "high-pressure high-temperature sintering" (HPHTS) that fuses/sinters 100% vulcanized rubber powder into a solid mass recovering approximately 35-40% of the original mechanical properties. A method of enhancing the mechanical properties of sintered natural rubber powder by incorporating various organic compounds will be discussed in this paper. Additives of different chemical structures such as dienophiles (maleic acid and maleic anhydride), dipolarophiles (phthalimide), and organic acids (benzoic acid, salicyclic acid, and others) are employed. Attenuated total reflection infrared spectroscopy (ATR-IR) and electron ionization mass spectrometry (EIMS) are used to investigate the mechanisms of sintering and the underlying factors behind the enhancement of the properties that is seen with incorporation of these additives.