By employing a combination of three-dimensional atom-probe tomography and first-principles calculations,significant qualitative and quantitative differences in solute segregation at coherent and semicoherent interfaces bounding a single q¹ precipitate in an Al-Cu-based alloy are found. Qualitatively, localized segregation is observed at the semicoherent interface, whereas delocalized behavior is present at
the coherent facets. Quantitatively, segregation at the semicoherent interface is a factor of 2 greater than at the coherent interface, resulting in a decrease in interfacial energy that is more than 5 times greater than that observed at the coherent facet. These observations illustrate unambiguously the strong couplings among interface structure, chemical composition, and energetics.