The pure rotational spectrum of FeO+ (X (6)Sigma(+)) has been recorded using millimeter/submillimeter velocity modulation techniques, the first high-resolution measurement of a metal oxide cation. FeO+ was created in an AC discharge from Fe(CO)(5) and N2O. Nine rotational transitions were recorded in the range 299-544 GHz, each consisting of six fine-structure components. The data were analyzed with a case (b) Hamiltonian, and rotational and spin parameters determined. The spin-spin constant suggests a low-lying (4)Sigma(-) excited state. The bond length established for FeO+, T-0 = 1.641 angstrom, is slightly longer than that of FeO, consistent with a covalent bonding scheme. (C) 2010 Elsevier B.V. All rights reserved.