Gold-copper alloy formation has been observed during gold cementation on copper from ammoniacal thiosulfate solution. The electrochemistry of this system is very complex and has required the use of multiple techniques to investigate the reaction fully. In general, cyclic voltammetry and linear sweep voltammetry yield very little information regarding the cathodic reduction of gold(I)-thiosulfate species on copper. Fortunately, the electrochemical quartz crystal nanobalance (EQCN) provides a useful method to determine the reduction potential of the various species. Linear sweep voltammetry was used to determine the reduction potential of both gold(I)-thiosulfate and copper complexes with EQCN. It was observed that gold(I)-thiosulfate is reduced at -250mV vs. SHE. The normal reduction potential of copper calculated by Nernst equation is -404 mV vs. SHE and the experimental value is -430 mV vs. SHE. When the solution contains 50 ppm [Au] and 80 ppm [Cu] initially, it was observed that copper reduced at -310 mV vs. SHE which is more positive than the theoretical potential. It confirms that copper deposition on gold in thiosulfate solution exhibits underpotential deposition behavior. The UPD phenomena can explain the characteristic formation of a series of gold-copper alloys.