In situ high-resolution transmission electron microscopy in the profile geometry has been used to observe the evolving features of the Au/InSb(111) B-(2 x 2) interface. During Au deposition in the range between 0 monolayer (ML) and similar to 1 ML coverage, the outermost Sb-trimer layer of the InSb(111) B-(2x2) substrate changes in contrast, presumably revealing that deposited Au atoms are partially captured into it. At similar to 2 ML coverage, an unknown phase emerges on the outermost layer, beyond which it continues to grow epitaxially in an island state, causing partial disruption of the substrate. The phase is identified as Au9In4 alloy with a gamma-brass structure determined from a digital Fourier transform diffractogram and a transmission electron diffraction pattern. The epitaxial relationship of Au9In4 with the substrate is given by (111)InSb parallel to(111) Au9In4 and [1(1) over bar 0] InSb parallel to[1(1) over bar 0] Au9In4. The high resolution-profile transmission electron microscopy images of this alloy agree well with the results calculated by the multislice method.