Examinations Of CO2 formed during steady-state CO oxidation reactions were performed using infrared (IR) chemiluminescence. The CO2 was formed using a molecular-beam method over Pd(1 10) and Pd(1 11). The CO2 formation rate is temperature dependent under various partial pressure conditions. The temperature of the maximum formation rate is denoted as T-Smax. Analyses of IR emission spectra at surface temperatures higher than T-S(max) showed that the average vibrational temperature (T VAI) is higher for Pd(1 11) than for Pd(110). The antisymmetric vibrational temperature (T VAS) is almost equal on both surfaces. These results suggest that the activated CO2 complex is more bent on Pd(1 11) and straighter on Pd(1 10). Furthermore, the difference in the T-V(AV) value was small for surface temperatures less than T-S(max). The T-V(AS) value was much higher than T-v(AV) on both surfaces. These phenomena were observed only when the surface temperature was lower than T-S(max): they became more pronounced at lower temperatures, suggesting that the activated complex Of CO2 formation is much straighter on both Pd surfaces than that observed at higher surface temperatures. Combined with kinetic results, the higher CO coverage at the lower surface temperatures is inferred to be related to the linear activated complex Of CO2 formation.