Nonformaldehyde, low pH (compared to highly alkaline bath) electroless copper plating has been investigated. Thiourea and its derivatives have been shown to increase the deposition rate of electroless copper plating solutions using HEDTA [N-(2-hydroxyethyl)ethylenediaminetriacetic acid trisodium salt hydrate] as the complexing agent and sodium hypophosphite as the reducing agent. A thiourea concentration of 1.0 ppm produced a fourfold increase in the deposition rate of copper from about 1 to 4 mum/h. The effect of thiourea on the electrochemical reactions, and the crystal structures and electrical properties of the copper deposits were examined. A small amount of thiourea, or its derivatives, in the electroless copper solution improves the catalytic activity of the copper surface for the oxidation of hypophosphite, resulting in a higher electroless deposition rate. The thiourea also increases the growth colony size of the copper deposits and improves its conductivity. A reaction mechanism is proposed to describe the function of the thiourea and its derivatives on the process.