The crystal structure of Mg(2)B(25), dimagnesium penta-eicosa-boride, was reexamined from single-crystal X-ray diffraction data. The structural model previously reported on the basis of powder X-ray diffraction data [Giunchi et al. (2006 ▶). Solid State Sci.8, 1202-1208] has been confirmed, although a much higher precision refinement was achieved, leading to much smaller standard uncertainties on bond lengths and refined occupancy factors. Moreover, all atoms were refined with anisotropic displacement parameters. Mg(2)B(25) crystallizes in the β-boron structure type and is isostructural with other rhombohedral compounds of the boron-rich metal boride family. Magnesium atoms are found in inter-stitial sites on special positions (two with site symmetry .m, one with .2 and one with 3m), all with partial occupancies.