EMF and CV measurements on ternary amalgams (Cu,Hg,Zn) were performed with Cu and Zn contents varying over a wide range, even above saturation. It was shown that the known interference between Cu and Zn in analytical chemistry is caused by an intermetallic compound, whose formation from gamma-CuHg and Zn is slow compared with the electrode kinetics of Zn and Cu af Hg. Its electrode potential is similar to 150 mV more postive than the Zn potential. The diffractogram of this phase leads to the conclusion that the intermetallic compound is the beta-phase "CuZn" distributed as nanocrystallites in Hg. Nanocrystallinity of "CuHg" is a prior condition for the interference to occur.