The work reported here principally involves a transmission electron microscopic examination of a thin oxide film formed on the surfaces of precipitates in aged AuFe alloys with Fe concentration 25 and 33 at.% on exposure to air at room temperature. By tilting through the reciprocal lattice, bright-field and dark-field image analysis, and double diffraction considerations led to a general picture of the surface layer. The following conclusions were drawn: (a) An Fe O oxide layer forms spontaneously on the surface of precipitates of aged AuFe specimens once the precipitates are exposed to air at room temperature. (b) The orientation relation between the oxide and a precipitate depends on the surface orientation. The Bain relation is obeyed when the precipitate surface is {100} . Both Bain and Nishiyama-Wasserman relations were found on other low index surfaces. (c) The oxide layer is composed of small, discrete particles that epitaxially grow on the surface of the precipitates. (d) Complicated diffraction patterns resulted from the surface oxide layer. The precipitates in AuFe alloys after prolonged aging are simply BCC α-Fe. Ordered precipitates were not found. © 1991.