The adsorption of aurous cyanide (AuCN) on the surface of Au(lll) from solutions containing KAu(CN)(2) has been investigated using cyclic voltammetry (CV), in-situ electrochemical scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The cyclic voltammogram exhibits a characteristic couple of reversible peaks at +0.15 V vs SCE attributable to the adsorption of AuCN. The in-situ STM study revealed two different structures for the AuCN adlayer, i.e., incommensurate structures of p(1.15x root 3R-30 degrees) and p(1.41x2 root 3R-30 degrees) on the positive and negative sides of the peak potential, respectively. The results of ex-situ LEED and AES analyses were consistent with the in-situ STM results. Furthermore, the reversible transformation between the two adlayer structures was consistently observed in potential scans near the peak. These results show that the well-ordered AuCN adlayers are formed at potentials 0.8 V more positive than the bulk gold deposition potential and that the adlayer structures are transformed reversibly at the phase transition peak. This study is believed to represent the first in-situ STM observation of the structures of AuCN adlayers formed from dicyanoaurate(I) anion, Au(CN)(2)(-).