Different Au/MgO catalysts modified with Mn, and Fe were prepared by precipitation-deposition. The catalysts were characterized by XRPD, FTIR and temperature programmed reduction (TPR) and were tested in CO oxidation in the temperature range of 243-543 K. The catalytic tests using both temperature programmed oxidation (TPO) of CO and time on stream (TOS) experiments revealed the high activity of catalysts prepared. In TPO of CO the modification by Mn significantly increased the activity above 300 K, however, in the sub-ambient temperature range the activity decreased considerably. Catalysts modified by iron did not show any decrease in the activity below 300 K, while the activity at high temperature increased significantly. Catalysts modified by Mn and Fe showed higher activity, selectivity and stability in preferential CO oxidation, carried out in the presence of hydrogen than the unmodified Au/MgO catalysts. Results of in situ FTIR spectroscopy on all catalysts unambiguously revealed the formation of ionic gold in the presence of CO/O-2 mixture. Results obtained in this study were interpreted by the formation of "metal ion-metal nanocluster" ensemble sites. These sites are involved in the activation of the CO molecule prior to its reaction with oxygen. It is suggested that at sub-ambient temperature these ensemble sites contain ionic forms of gold, while at higher temperature probably iron or manganese ions are involved in CO activation. (C) 2004 Elsevier B.V. All rights reserved.