Reactions of small neutral iron oxide clusters (FeO1-3 and Fe2O4,5) with carbon monoxide (CO) are investigated by experiments and first-principle calculations. The iron oxide clusters are generated by reaction of laser-ablation-generated iron plasma with O-2 in a supersonic expansion and are reacted with carbon monoxide in a fast flow reactor. Detection of the neutral clusters is through ionization with vacuum UV laser (118 nm) radiation and time-of-flight mass spectrometry. The FeO2 and FeO3 neutral clusters are reactive toward CO, whereas Fe2O4, Fe2O5, and possibly FeO are not reactive. A higher reactivity for FeO2 [sigma(FeO2 + CO) > 3 x 10(-17) cm(2)] than for FeO3 [sigma(FeO3 + CO) similar to 1 x 10(-17) cm(2)] is observed. Density functional theory (DFT) calculations are carried out to interpret the experimental observations and to generate the reaction mechanisms. The reaction pathways with negative or very small overall barriers are identified for CO oxidation by FeO2 and FeO3. The lower reactivity of FeO3 with respect to FeO2 may be related to a spin inversion process present in the reaction of FeO3 with CO. Significant reaction barriers are calculated for the reactions of, FeO and Fe2O4-5 with CO. The DFT results are in good agreement with experimental observations. Molecular-level reaction mechanisms for CO oxidation by O-2, facilitated by condensed phase iron oxides as catalysts, are suggested.