The reaction mechanism between a Pd/Sn catalyst and sulfide ions which were used to promote direct copper plating was studied. The identity of reaction products after activation, acceleration, sulfide promotion, and plating bath dipping were analyzed by infrared absorption spectroscopy and X-ray diffraction. It wa found that the chemically adsorbed sulfides ions do not readily form PdS. The stable PdS appears only after it is dipped in the plating bath. The major agent responsible for this transformation was found to be the sulfate ions.