An adhesive composition comprising a modified, ethylene/ a-olefin copolymer rubber or a modified, ethylene/a-olefin/non-conjugated diene copolymer rubber containing 0.1 - 100/o by weight of at least one member selected from the group consisting of an acid anhydride and a dicarboxylic acid, and, if necessary, as an amine compound,

(i) 0.2 - 2 moles of a dialkylamine per mole of the acid anhydride in said modified copolymer rubber when said modified copolymer rubber contains no dicarboxylic acid;
(ii) 0.2 - 2 moles of a polyamine compound per mole of the total amount of the acid anhydride and the dicarboxylic acid in said modified copolymer rubber; or
(iii) 0.2 - 2 moles of a dialkylamine per mole of the acid anhydride in said modified copolymer rubber and 0.2 - 2 moles of a polyamine compound per mole of the acid anhydride in said modified copolymer rubber when said modified copolymer rubber contains no dicarboxylic acid. The above adhesive composition exhibits good adhesion for rubber compositions, particularly low saturated, non-polar rubber compositions and fibers, and is excellent in storage stability.