The morphology, electron structure and catalytic activity in CO oxidation of the following samples were compared: an Au/FeOx/SiO2/Si(1 0 0) model sample fabricated by pulsed laser deposition (AuPLD), an Au/Fe2O3 sample prepared by co-precipitation (AuCP) and an Au-Fe/HY (AuHY). It was established that oxygen treatment at 470 K for 1 h increases the rate of CO oxidation on AuPLD and AuCP samples as compared to subsequent hydrogen treatment at 470 K for 1 h, whereas on sample AuHY the effect is reversed. The difference cannot be ascribed to the change in particle size because the average diameter of gold particles is in the range 4-6 nm, which is only slightly modified by oxygen/hydrogen treatments. The difference among the samples cannot be ascribed to surface carbon contamination, because after oxygen treatment the amount of carbon slightly decreases and this cannot account for the activity increase. The major difference lies in the gold/iron oxide interface, which is well developed in the case of AuPLD and AuCP, but it does not exist inside the zeolite in the AuHY sample. In the latter case, the O-2(-) superoxide which is responsible for the enhanced activity cannot be formed. A possible mechanism is given in this paper.