Three-dimensional coherent diffraction patterns of an isolated, single-crystalline Ag/Au core-shell nanowire were recorded at different X-ray beam energies close to the Au LIII absorption edge. Two-dimensional slices of the three-dimensional diffraction pattern, with the diffraction vector oriented perpendicular to the wire axis, were investigated in detail. In reciprocal space, facet streaks with thickness fringes were clearly observed in the two-dimensional diffraction patterns, from which the shape and size of the corresponding cross sections of the nanowire could be revealed. Comparison with simulated diffraction patterns exhibited the coherency strain field in the nanowire. During in situ annealing at temperatures which would lead to significant intermixing by volume diffusion in bulk material, according to literature data, a core-shell morphology was preserved; that is, intermixing in the nanowire was pronouncedly decelerated compared to bulk diffusion.