A new bonding system is described in an attempt to improve the adhesion stability of brass-plated steel cord to rubber without deterioration of initial adhesion performance. Two brass/cobalt/copper-plated steel cords with different cobalt layer thicknesses were prepared, and their adhesion performance including adhesion stability to rubber compound were measured and compared with those of conventional brass-plated steel cords. It was found that the new cords showed not only an excellent adhesion stability against aging conditions, notably humidity aging, but also a superior initial adhesion performance. The excellent adhesion stability was explained by the suppressed dezincification reaction of brass by cobalt layer.