The interface structures resulting from the alloying reactions between a Au/Ni/Au‐Ge composite film and a (100) GaAs substrate were studied by transmission electron microscopy and scanning transmission electron microscopy. Electron microscope examinations of the cross‐sectional samples prepared in this study offered excellent lateral and depth resolution of local structures which are not available by other analytical techniques used previously in similar studies. The distributions and chemical compositions of various phases formed, and the morphologies of the interfaces between these phases were monitored and compared with the measured contact resistances at three different stages of alloying. A correlation between the interface structure and the contact resistance was found.