OC-Cu-I, previously uncharacterised in the gas phase, was generated by the reaction of laser-ablated Cu with methyl iodide contained in a mixture of argon and CO. The Cu-I initially formed picks up CO to yield OC-Cu-I. The rotational spectrum was observed by means of a pulsed-jet, Fourier-transform microwave spectrometer. The rotational constants B-0, centrifugal distortion constants D-0, nuclear quadrupole coupling constants and spin-rotation coupling constants for Cu and I were determined for the four isotopomers (OC)-O-16-C-12-Cu-63-I-127, (OC)-O-16-C-12-Cu-65-I-127, (OC)-O-18-C-12-Cu-63-I-127 and (OC)-O-16-C-13-Cu-63-I-127. Geometries of the r(0), r(s), and r(m) type are presented and changes in the electronic structure when OC-Cu-I is formed from Cu-I are discussed. (c) 2006 Elsevier B.V. All rights reserved.