The invention comprises a vulcanizable composition comprising a sulfur vulcanizable rubber, a sulfur vulcanizing agent, an accelerating agent in an amount effective to accelerate vulcanization and sufficient to not substantially inhibit vulcanization, and an amount effective to improve the hardness stabilization of said rubber upon vulcanization of a pyrimidine derivative. The invention also comprises a method of improving the hardness stabilization of a composition comprising a sulfur vulcanizable rubber.