The effects of thiourea and guanidine hydrochloride on electroless copper plating were studied by voltammetric and gravimetric methods. Thiourea stabilizes the plating bath and simultaneously decreases the plating rate. The effect of stabilization is attributed to the strong complexing ability of thiourea with Cu(I) and the surface adsorption of thiourea on copper. The surface adsorption is the main reason for the decrease in plating rate. In the case of guanidine hydrochloride, the plating rate is maximum when its concentration is approximately 5 mg/liter. The occurrence of the maximum is interpreted in terms of the interplay of the delocalized π electron effect of guanidine hydrochloride and its surface adsorption on copper.