The effect of cobalt salt addition on the adhesion between rubber compound and brass-plated steel cord was studied using cobalt boroacylate, cobalt naphthenate and cobalt stearate to discuss the function of cobalt salt as an adhesion promoter. The composition and state of the adhesion layer at rubber/brass thin-film sample were also investigated. The unaged adhesion properties were improved by addition of cobalt salt, but the further improvement was not observed with increase in the loading of cobalt salt above 0.16 phr as metal. The aged adhesion properties of the rubber compound with high levels of cobalt salt was poor compared with those of cobalt salt-free rubber compound. The decreasing behavior in the adhesion properties with aging treatment varied with the anion species of cobalt salt. The rubber compound with low loading of cobalt boroacylate as 0.16 phr showed the best adhesion properties at unaged state as well as after thermal, humid, and salt solution aging treatments. The acceleration of copper sulfide formation with the addition of cobalt salt and excessive formation of copper sulfide with high levels of cobalt salt were observed from rubber/brass thin-film samples. The promotion role of cobalt salt in the adhesion might be summarized as the synergy effects due to the acceleration of the formation of copper sulfide at the adhesion interface and the enhancement of the crosslinking density of rubber compound attached to the adhesion layer.