The influence of reducing conditions on transient redox behavior of a series of noble metal (NM)-loaded Ce0.68Zr0.32O2 and CeO2 materials was investigated. When H-2 was used as a reducing agent, significant dynamic oxygen storage (H-2-OSC) values, with H2O formation, were measured at room temperature (RT). This is attributed to H-2 spilled over the support followed by titration of this adsorbed hydrogen by O-2. The phenomenon requires the presence of metal to activate H-2, and was observed for Rh, Pd, and Pt, all of which show high efficiency in this regard. Furthermore, there appears to be a direct relationship between this process and surface area: Decreasing the surface area of samples diminishes RT H-2-OSC proportionally. The higher thermal stability of ceria-zirconia relative to ceria means that H-2-OSC remains significant even after severe redox aging. Over Pt/CeO2, the H-2-OSC values measured at 373-773 K depend on surface area, indicating that the reduction is a surface-related process, whereas for ceria- zirconia-supported samples deeper reduction/vacancy creation becomes evident as the temperature is increased, the dynamic H- 2-OSC being therefore independent of sample surface area. When CO is used as reducing agent, complicated dynamic CO-OSC behavior is observed, with surface reduction, CO storage, and CO desorption being detected. For the fresh (oxidized) high- surface-area Pt/Ce0.68Zr0.32O2 catalyst, appreciable CO-OSC is detected only at and above 473 K. When the catalyst is prereduced at 500 K, CO-OSC is observed at 373 K. In contrast, no appreciable CO-OSC was detected over law-surface-area samples below 673 K. Compared with Pd and Pt, which exhibit similar behavior, the presence of Rh promotes support reduction by CO. (C) 2001 Academic Press