Liquid-phase in situ XANES experiments were used to elucidate the oxidation state of differently supported Au catalysts during the selective oxidation of I-phenylethanol with molecular oxygen. The catalysts investigated included (i) Au(0.6 wt%)/Cu3Mg3Al2Ox, (ii) Au(0.6 wt%)/Mg3AlOx,(iii) Au(2.9 wt%)/CeO2, and (iv) a copper-doped analog, Au(0.6 wt%)/CuO-CeO2 (20 wt% CuO). The XANES and EXAFS data were collected mainly in the fluorescence mode. Simultaneous determination of the structure (XANES) and the activity of the catalysts in the continuous-flow microreactor was achieved by following the oxidation state using XANES and the characteristic carbonyl vibration of the desired reaction product acetophenone using an IR spectrometer equipped with a transmission cell. Au/Cu3Mg3Al2O, showed a small increase of conversion with time on stream accompanied by a slight reduction of the gold component. With Au/mg(3)AlO(x), and Au/CuO-CeO2, a stronger reduction accompanied by increased catalytic activity was observed. The results of these mixed oxide-supported Au catalysts demonstrate that metallic gold is the main active species in catalytic aerobic alcohol oxidation. In contrast, a decrease in the product formation rate was observed for Au/CeO2 with ongoing reduction. Results obtained from factor analysis indicate that the decrease in activity of the Au/CeO2 with time on stream does not correlate with the much faster reduction of the catalyst and thus cannot be attributed to the increasing fraction of Au-0 species. This finding corroborates the importance of recording XANES spectra for structural identification simultaneously with online determination of the catalytic performance. (c) 2007 Elsevier Inc. All rights reserved.