X-ray diffraction (XRD) pattern of nanosized equimolar solid solution CoIr prepared by thermolysis of [Co(NH3)6][Ir(C2O4)3] contains peaks characteristic of both face-centered cubic (fcc) and hexagonal close-packed (hcp) structure. Moreover, 101 peak of hcp modification is substantially wider than 100 and 002 peaks, 102 and 103 are very broad and almost invisible. Peak 200 of fcc structure is wider than the other peaks of this modification and slightly shifted toward lower angles. It was shown by simulation of XRD patterns that particles of CoIr alloy are nanoheterogeneous and consist of lamellar domains having fcc and hcp structures. The best fit was obtained for the following model parameters: An average crystallites size is about 10 nm, average thicknesses of the fcc and hcp domains are 1.7 and 1.1 respectively. The presence of domain structure was confirmed by transmission electron microscopy data.