The infrared chemiluminescence study of CO, formed during steady-state CO + O-2 reaction over Pd(1 1 0) under various CO/O-2, pressure ratios was carried out. Highly excited antisymmetric vibrational mode compared to bending vibrational mode was observed under reaction conditions at high CO coverage. This can be related to the activated complex of CO, formation in more linear form. (c) 2005 Elsevier B.V. All rights reserved.