Abstract: Copper etchant solution additives for use with an aqueous alkaline ammoniacal cupric chloride etching bath include several compounds, each of which is shown to stabilize the copper(I) state. The compounds discovered by the present invention include iodide ions such as potassium iodide, ammonium iodide, sodium iodide, calcium iodide and magnesium iodide. Other copper(I) stabilizers discovered by the present invention include certain water soluble salts containing sulfur such as a thiocyanate ion (e.g. ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, magnesium thiocyanate, and calcium thiocyanate) and a thiosulfate ion (e.g. ammonium thiosulfate, potassium thiosulfate, sodium thiosulfate, magnesium thiosulfate, and calcium thiosulfate). Etching rates for alkaline ammoniacal cupric chloride with different concentrations of potassium iodide, ammonium thiocyanate, and sodium thiosulfate were studied.