The aim of this study was to determine the influence of the N-isopropyl-N0-phenyl-p -phenylenediamine (IPPD) antioxidant agent on degradation of composites NBR/SiO 2. The inhibition of the early stages of the degrading process has been particularly analyzed. Several NBR-SiO 2  matrix samples with and without IPPD have been exposed to an oxidizing atmosphere at 140  C for periods between 0 and 120 h. Besides, IPPD alone has been subject to the same aging thermo-oxidizing process. The degraded samples have been characterized by ATR-FTIR, TG, and SEM. Mechanical properties and degree of crosslinking have been determined. The results show that the IPPD improves the mechanical behavior of the composites submitted to thermo-oxidization, increasing the time in which composite retains rather good mechanical properties. IPPD decreases the ability of crosslinking of the rubber. ATR-FTIR spectroscopy study shows that the degradation process increases the volatility of the plasticizer and generates new unsaturation sites, which later will be the nuclei of crosslinking.