PURPOSE: To obtain a vulcanizable rubber compsn. which gives a vulcanizate with excellent mechanical strengths and a good modulus.

CONSTITUTION: A mixture comprising 100 pts.wt. metal salt of an ethylenically unsatd. carboxylic acid, 0.01-10 pts.wt. org. peroxide, and 70-300 pts.wt. inert flowable medium is thermally treated to give a compsn. contg. a cross-linked polymer formed from the metal salt. A vulcanizable rubber compsn. is prepd. by compounding 100 pts.wt. unsatd. rubber with 5-100 pts.wt. above-mentioned compsn. contg. the cross-linked polymer and 0.2-10 pts.wt. sulfur-based or org.- peroxide-based vulcanizing agent.