Theoretical and experimental studies have been made on the separation of trace amounts of copper by reduction and adsorption on mercury metal in thiocyanate media. This method was studied experimentally with the radioisotope ⁶⁴Cu and is based on the adsorption of the Cu(I)-SCN complex. The factors limiting the extractions are described. There is a competitive adsorption between the complexes of CuSCN and Hg(SCN)₂ under certain conditions. The method can be applied in fast radiochemical separations for copper contents in the range 10^-5–10^-9 g ml^-1.