Abstract: A stabilized polyisoprene composition comprising 100 parts by weight of polyisoprene and 0.15-3.0 parts by weight of a binary system antioxidant consisting of 50-99% by weight of a phosphite type antioxidant represented by the formula, ##STR1## wherein R.sub.1, R.sub.2, and R.sub.3 are same or different aryl groups, and 50-1% by weight of a triazine derivative type antioxidant represented by the formula, ##STR2## wherein X.sub.1, X.sub.2, and X.sub.3, which may be the same or different, are selected from the group consisting of --NH--, --O--, and --S--, and R', R", and R"', which may be the same or different, represent alkyl, or alkyl-substituted or unsubstituted hydroxyphenyl groups. This binary system antioxidant exhibits a synergistic effect on the stabilization of polyisoprene against heat and oxygen.