Abstract Dynamic studies of CO oxidation have shown complex behavior such as rate enhancement and CO 2 peak formation during forced-periodic operation. The transient chemical processes which account for this complex behavior during dynamic operation have not been fully identified, indicating the need for further study. In this work, two Pt/Al 2O 3 catalysts, one prepared from a chloridecontaining precursor (chloroplatinic acid) and one prepared from a chloride-free precursor (tetraamineplatinum nitrate), were examined in terms of their steady-state reaction behavior and dynamic responses to periodic changes of CO pressure in O 2. During reaction experiments, temperatures ranged from 90 to 190°C, CO pressures ranged from 0 to 0.2 Torr at a constant O 2 pressure of 0.33 Torr, and cycling periods ranged from 5 to 24 s. In addition, adsorption calorimetry and temperature programmed desorption (TPD) experiments were performed to characterize CO adsorption over the catalysts. The results of the steady-state and dynamic reaction experiments correlated with each other and with the adsorption and TPD results and indicated that the sample prepared from tetraamineplatinum nitrate had stronger CO-Pt interactions than the sample prepared from chloroplatinic acid. The two catalysts had similar Pt dispersions, and this comparison shows that measurement of metal dispersion is not sufficient to characterize a supported metal catalyst for CO oxidation. Addition of HCl to the chloride-free catalyst did not have a significant effect on CO adsorption as indicated by TPD. The complex dynamic responses measured over both catalysts demonstrate that transient processes on the catalyst surfaces produced CO 2 at higher rates than would be predicted by instantaneous response to steady-state conditions during dynamic operation.