The effect of similar to0.1 monolayer of adsorbed S on the electrochemical response of polycrystalline Cu in a phosphate buffer electrolyte at pH 6.4 has been studied using combined ultrahigh vacuum electrochemistry methodology. This study shows that the anodic dissolution current is higher for both S-free Cu and S-modified Cu electrodes at pH 6.4, when compared to dissolution currents observed in a previous study in a borate buffer at pH 8.4. The absence of CuO in the surface oxide-hydroxide layer for electrodes subjected to anodic dissolution in the phosphate buffer is proposed to be the main reason for enhanced dissolution currents observed at this pH. The presence of adsorbed S does not have a significant effect on anodic dissolution contrary to the behavior observed in the previous study in borate buffer. Adsorbed S is found to inhibit oxide-hydroxide formation at the peak dissolution potential.