Nitrosonium pentanitratoholmate(III), (NO)₂Ho(NO₃)₅, was prepared from holmium metal and dinitrogen tetroxide in anhydrous ethyl acetate. Its crystals are monoclinic with a = 8.094, b = 11.979, c = 14.170 Å and β = 104.7°,Z = 4, space group P2₁/c. The structure was solved by the heavy-atom method and refined by full matrix least-squares methods to an R value of 0.085 for 1514 observed reflections measured on the G.E. XRD-6 diffractometer. The structure consists of one Ho(NO₃)₅²⁻ and two NO⁺ ions. The holmium is 10-coordinated by five essentially symmetric bidentate nitrate groups which arrange themselves in a trigonal bipyramidal fashion around the metal with a mean Ho—O distance of 2.45 Å. The two NO⁺ ions per metal occupy slightly different environments and give rise to two i.r. bands at 2250 and 2285 cm⁻¹, attributable to the (N, O⁺) stretching vibration. The mean N—O distance within these ions is 1.00 Å.