The aim of this study was to determine the influence of the N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD) antioxidant agent on degradation of composites NBR/SiO2. The inhibition of the early stages of the degrading process has been particularly analyzed. Several NBR-SiO2 matrix samples with and without IPPD have been exposed to an oxidizing atmosphere at 140 degrees 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. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2011-2018, 2009