We have conducted a detailed investigation of the initial stages of the formation of metal nanoparticles during the incipient wetness synthesis process for both Co on gamma-Al2O3 and Ru-promoted Co on gamma-Al2O3 catalysts. The synthesis was performed in the reaction cell of an environmental transmission electron microscope so that the processes can be followed using in situ atomic resolution imaging, Z-contrast imaging and nanospectroscopy. The metal precursors were found to have a highly non-uniform distribution on the alumina support, which leads to a non-uniform distribution of metal particles in the final catalysts. In situ atomic resolution images and electron energy-loss nano-analyses showed that during the reduction process, the metal precursor material transformed into an intermediate phase of cubic Coo. With the addition of Ru, however, in addition to the presence of large Coo particles, small particles of either CoRu or pure Ru were also formed. Our experimental results directly show that the addition of Ru promotes the formation of individual CoRu bimetallic nanoparticles and enhanced the reducibility of small Co particles in the Co(Ru)/gamma-Al2O3 systems. (c) 2006 Elsevier B.V. All rights reserved.