In search of the cause of a copper deposit formed in some incandescent lamps with copper electrodes, we examined the thermal decomposition of some organic copper salts. Cuprioxalate yields copper oxides, and cuprimesoxalate a mixture of copper oxides with elementary carbon. Cupriformate, however, decomposes at about 200°C., evolving partially a volatile compound of copper which immediately decomposes, forming a red smoke of elementary copper, and coating the near hot glass surface with a brilliant mirror of metallic copper. Where this mirror was thin, it resembled in every way the red deposit found in incandescent lamps.