The surface morphology, composition, and structure of Au3Cu(001) as determined by scanning tunneling microscopy and surface x-ray diffraction are presented. Atomic resolution STM images reveal distinctive geometric features. The analysis of the surface x-ray diffraction data provides clear evidence of gold enrichment of the topmost layer with two different gold positions and only a small copper fraction. The second layer has approximately Cu4Au stoichiometry, whereas the third layer is pure gold. STM images and SXRD results are in excellent agreement and reveal a detailed atomic model for the surface structure. [S0163-1829(99)04535-X].