Cytochrome oxidase is the terminal oxidase in both prokaryotic and eukaryotic cells and is responsible for the generation of cellular energy via the process known as oxidative phosphorylation. The enzyme contains two Fe and three Cu centers which together provide the redox machinery for the reduction of O2 to water. Recently, X-ray crystallography has provided the first three-dimensional description of the coordination spheres of the metal centers. However, the structures show the metal sites at low resolution, and in order to fully understand the mechanism of the reaction, it is desirable to determine the metrical details (bond lengths and angles) to much higher precision. X-ray absorption spectroscopy is unique in its ability to provide such detail, and we have applied the technique to determining the structure of the CuA center, a thiolate-bridged binuclear copper cluster in which the coppers are bridged by two cysteine ligands and have an extremely short Cu−Cu distance of ∼2.4 A. X-ray absorption spec...