Electronic properties of molecules and carbon nanomaterials are usually affected by metal substrates. An electronic decoupling buffer layer is of importance to reveal their intrinsic properties. Here, the strength of electronic decoupling by a gold silicide buffer layer formed on Au(111) was studied using scanning tunneling microscopy/spectroscopy. The HOMO–LUMO gap of fullerene adsorbed on the buffer layer is approximately 3.0 eV, which is in between that on bare Au(111) and on a NaCl bilayer film, indicating a moderate decoupling.