The pure rotational spectrum of AuO in the (XII3/2)-I-2 state was observed by employing a source-modulated microwave spectrometer. The AuO radical was generated in a free space cell by the sputtering reaction from a gold sheet lining the inner surface of a stainless steel cathode using a do glow plasma of O-2 and Ar. Observation of the rotational transitions in both the ground and first excited vibrational states enabled to determine molecular constants by a least-squares fit. Other spectroscopic parameters such as equilibrium bond length, r(e), and vibrational wavenumber, omega(e), were also derived from the obtained molecular constants. (C) 2005 Elsevier B.V. All rights reserved.