We present in-situ IR (DRIFTS) measurements on CO adsorption and preferential CO oxidation (PROX) in H-2-rich gas on Pt/gamma-Al2O3 and Au/alpha-Fe2O3 catalysts at their envisaged operating temperatures of 200 degrees C and 80 degrees C, respectively, which in combination with kinetic data show that the underlying reason for the very different PROX reaction kinetics on these two catalysts is the difference in steady-state CO coverage. Whereas on the platinum catalyst this is always near saturation under reaction conditions, causing a negative reaction order (- 0.4) and a p(CO)-independent selectivity, the amount of adsorbed CO on the gold particles (indicated by an IR band at similar to 2110 cm(-1)) strongly depends on the CO partial pressure. From the position of the IR band of CO adsorbed on Au/alpha-Fe2O3, the steady-state coverages on the Au surface are shown to be significantly below saturation, with an upper limit of approximately theta(CO) = 0.2. Low reactant surface concentrations on Au explain the positive reaction order with respect to p(CO) (+0.55 at 80 degrees C) as well as the observed decoupling of the CO and H-2 oxidation rates, which results in a loss of selectivity with decreasing p(CO).