The adhesion between tetrachlorobenzoquinone (TCBQ)-loaded rubber compound and brass-plated steel cord was studied to understand the role of TCBQ as an adhesion promoter. With loading of TCBQ in the rubber compound the cure rate became slow, but the change in physical properties was not significant. An improvement of the adhesion was obvious with low loading of TCBQ at the range of 0.5 phr while an adverse effect was observed with high loading at 2 phr. The addition of TCBQ for the improvement of adhesion has a greater effect before aging because TCBQ contributes to form a large surface area of copper sulfide via accelerated copper diffusion into the rubber bulk. However, higher TCBQ loading as well as longer aging time leads to a great extent of copper migration into the rubber and excess growth of zinc oxide layer, resulting in decreased adhesion. The adhesion interphase before aging, between a thin brass him and a rubber compound, investigated using AES showed no difference except that sulfur increased with increasing TCBQ loading. On the other hand, the adhesion interphase was changed after humidity aging. The copper content decreased at the adhesion interphase while it increased in the rubber layer with increasing addition of TCBQ.