Nanocrystallite iron oxide powders with different crystallite sizes were prepared by co-precipitation route. The prepared powders with crystallite size 75, 100 and 150 nm together with commercial iron oxide (250 nm) were tested for the catalytic oxidation of CO to CO, The influence of different factors as crystallite size, catalytic temperature and weight of catalyst on the rate of catalytic reaction was investigated using advanced quadrupole mass gas analyzer system. It can be reported that the rate of conversion of CO to CO, increased by increasing catalytic temperature and decreasing crystallite size of the prepared powders. The experimental results show that nanocrystallite iron oxide powders with crystallite size 75 nm can be recommended as a promising catalyst for CO oxidation at 500 degrees C where 98% of CO is converted to CO2. The mechanism of the catalytic oxidation reaction was investigated by comparing the CO catalytic oxidation data in the absence and presence of oxygen. The reaction which was found to be first order with respect to CO is probably proceeded by adsorption mechanism where the reactants are adsorbed on the surface of the catalyst with breaking O-O bonds, then CO pick up the dissociated O atom forming CO2. (c) 2006 Elsevier Ltd. All rights reserved.