The surface-extended X-ray absorption fine structure (SEXAFS) spectrum of an electrochemically deposited adlayer of copper on a gold(111) electrode immersed in solution and under potential control was obtained by employing fluorescence detection and grazing incidence geometry. The X-rays were polarized parallel to the plane of the gold surface. Two peaks were observed in the radial distribution function, indicating two different near neighbors for copper. The near neighbors were determined to be gold at 2.58 ± 0.03 Å. and copper at 2.92 ± 0.03 Å. The copper-copper distance is identical with the gold-gold lattice spacing, showing that at full coverage the copper adlayer forms a 1X1 commensurate layer on the gold(111) substrate. The intensity of the gold-copper scattering relative to the copper-copper scattering suggests the copper atoms sit in 3-fold hollow sites. © 1988, American Chemical Society. All rights reserved.