The mechanism of copper deposition by direct plating on a nonconductive substrate has been investigated. Studies using electron spectroscopic chemical analysis show that the immersion of a sodium sulfide solution does lead to the formation of palladium sulfide. Sulfide acts as a bridging Ligand; it can increase the speed of direct plating dramatically. Compounds with similar bridging property were later found to show the Same promotion effect in the direct plating process. These compounds include thiourea, sodium thiocyanate, potassium iodide, and sodium cyanide. It was also found that the addition of these compounds could cause a shift of the rest potential. In addition, copper was found to partially dissolve if the current was interrupted. These phenomena can all be explained by the proposed mechanism.