Mononuclear gold complexes supported on MgO powder were prepared by adsorption of Au(CH3)(2)(C5H7O2) and tested as catalysts for ethylene hydrogenation at atmospheric pressure and 353 K. The catalyst was characterized in the working state with extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and infrared (IR) spectroscopies. EXAFS and XANES data demonstrate the presence of mononuclear Au(III) complexes as the predominant surface gold species during catalysis, with no evidence of metallic gold clusters, confirmed by the lack of Au-Au contributions in the EXAFS spectra and the presence of the characteristic features of Au(III) in the XANES spectra. The mononuclear gold complex is bonded to two oxygen atoms of the MgO surface at an Au-0 distance of 2.16 Angstrom. Ethyl and pi-bonded ethylene on the mononuclear gold were identified by IR spectroscopy, with the former identified as a reactive intermediate and the latter apparently a spectator or involved as an intermediate in virtually equilibrated steps. Hydrogen on the mononuclear gold is inferred to form by dissociative adsorption of H-2, as indicated by the order of the catalytic reaction in H-2 of 0.5. (C) 2004 Elsevier Inc. All rights reserved.