The adhesion between a boric acid ester (BAE) containing rubber compound and a brass-plated steel cord was investigated to understand the role of BAE as an adhesion promoter, An improvement in adhesion was shown with the low loading of BAE in the range 0.5-1 phr, while significant decline of adhesion was not observed with high loading at 2 phr and a long aging time of 15 days. The interphase between the brass plated steel cord and the rubber compound studied using Auger electron spectroscopy (AES) showed a stabilized adhesion interphase by BAE incorporation, resulting in the enhancement of the adhesion retention.