Influences of different iron oxides, used as gold supports, on CO oxidation were tested by FT-IR spectroscopy during adsorption of CO in a vacuum and by DRIFTS spectroscopy in the reaction conditions. The two techniques yielded different results but their recombination led to very valuable and interesting conclusions. The investigation showed that the adsorption and oxidation are independent of chemical composition and structure of the supports, but that they depend only on presence of surface -OH groups. Also, no correlation was observed between the catalysts with -OH groups, which showed almost equal high activity, and the gold particles less than 5 nm. Namely, the particles with 1.5 nm did not adsorb CO at all, while particles with 4 nm showed three suitable sites for CO adsorption (2172, 2115 and 2071 cm(-1)). The catalyst without -OH groups had significantly less activity despite the fact that it is almost identical in chemical composition and structure to one of the active catalyst. (c) 2006 Elsevier B.V. All rights reserved.