The crystallographic structure of (11 wt.%)CuO-(6 wt.%)CeO2/gamma-Al2O3 has been studied and compared with (11 wt.%)CuO/gamma-Al2O3 under reducing conditions, using time-resolved in situ X-ray diffraction in the temperature range 25-800 A degrees C. In CuO-CeO2/Al2O3, H-2-TPR reduces the CuO phase to Cu, while in C3H8-TPR reduction follows a two-step pathway via Cu2O. A thermal treatment in He also induces reduction for CuO, albeit at higher temperature. In addition to CuO reduction, the CeO2 promoter in CuO-CeO2/Al2O3 is also partially reduced, without crystallographic transition, regardless of the atmosphere and at similar temperature where reduction of CuO occurs. Supported CuO as in CuO-CeO2/Al2O3 or CuO/Al2O3, is more readily reduced by thermal treatment in He than unsupported CuO and Cu2O. Moreover, the addition of CeO2 to the CuO-CeO2/Al2O3 catalyst allows for enhanced reducibility of CuO, compared to CuO/Al2O3. The CuO phase in CuO-CeO2/Al2O3 is reduced to Cu2O and partly to Cu at 700 A degrees C and mainly to Cu at 800 A degrees C in He flow. The thermal reduction of CuO-CeO2/Al2O3 requires an apparent activation energy of 216 kJ/mol. An isothermal reduction treatment at 800 C-o in He reduces CuO-CeO2/Al2O3, as demonstrated by time-resolved in situ X-ray diffraction. Supported CuO are more easily reduced by thermal treatment compared to unsupported CuO and Cu2O. The CuO phase in CuO-CeO2/Al2O3 is reduced to Cu2O and partly to Cu at 700 A degrees C and mainly to Cu at 800 A degrees C in He flow (see figure).