Iron oxide-supported gold samples were prepared by co-precipitation from HAuCl4 and Fe(NO3)3. The activities of the samples as CO oxidation catalysts were tested without thermal treatment and following treatments in flows of He and O2 at various temperatures. It was found that the untreated samples and those treated in a flow of He at 150 °C were more active than samples that had been treated at 400 °C in either a flow of O2 or of He. Infrared spectra recorded during CO oxidation catalysis indicate the presence of bonded CO molecules to cationic gold on all samples, whereas spectra of the least active catalysts indicate a predominant presence of Fe2+ carbonyls, which were highly stable under the conditions of our experiments. Our results indicate that in the least active samples the Fe2+-bound CO blocks sites that would otherwise be available for oxygen activation.