In situ scanning tunneling microscopy was employed to examine the interfacial structure of a well-defined Pt(111) electrode in both acidic and alkaline solutions containing NaCN or KCN. A well-ordered (2 root 3x2 root 3)R30 degrees CN-adlayer structure was observed at potentials in the double layer charging region. The adlayer consists of 6-membered-ring unit cells with cations coordinated in the center of the hexagon. The corrugation height of the center spot was found to be dependent on imaging conditions, becoming invisible under certain conditions, suggesting the absence of cyanide coordinated in the hexagonal ring. Bright spots were clearly observed in the center of the hexagonal ring when K+ cations were introduced into the solution. These potential-dependent bright spots were attributed to strongly coordinated K+ in the hexagonal ring of the adsorbed (CN on Pt(111).