MgO-supported gold catalysts modified by V, Pb, Sm, La and Y have been investigated in order to elucidate the role of modifiers in preferential CO oxidation. All catalysts have been prepared by means of homogeneous precipitation of nitrate precursors using urea as a precipitating agent. Catalytic properties and characteristic features of catalysts were investigated in detail. Different spectroscopic techniques (XPS, FTIR and UV-Vis) and HRTEM were applied for catalyst characterization. The coverage of gold nanoparticles by small MgO clusters has been evidenced. Despite the high MgO coverage of gold, the parent Au/MgO catalysts showed high activity in CO oxidation indicating the beneficial effect of MgO in atomic closeness to Au. It has been revealed that in Pb-modified catalysts, gold has been alloyed with Pb. The results definitely indicate that Pb is responsible for the decrease in the number of gold sites involved in the activation of hydrogen. It has been shown that in multi-component catalysts MgO is decorated by nanoclusters of different metal oxides, and the Pb-Au nanoparticles are located in intimate contact to these oxides. In multi-component catalysts, the interfaces formed in this way provide new type of active site ensembles that can adsorb and activate CO and oxygen, while suppressing the activation of hydrogen. (C) 2009 Elsevier Inc. All rights reserved.