The influence of Ce/Zr ratio on the redox behavior of Ni in a series of NiO-CeO2-ZrO2 catalysts was investigated using in situ electron paramagnetic resonance (EPR), diffuse reflectance UV-visible (DRUV-visible) and X-ray photoelectron spectroscopy (XPS). At all concentrations, a small amount of Ni (species I) substitutes in the fluorite lattice. Superparamagnetic, nanosize Ni crystallites (species II) were found in samples with 1-5 wt.% NiO and ferromagnetic, larger Ni crystallites (species III) were detected in samples with 20 wt.% or more NiO when contacted with hydrogen. Cc promoted the reduction of Ni. The reducibility of Ni decreased in the order: I > III > II. At steam reforming conditions (in the presence of H-2 + H2O + hydrocarbon/alcohol at 773 K), the extent of Ni reduction varies in the order: H-2 + alkane > H-2 + alcohol > H-2 alone. Catalytic activity and especially stability in the steam reforming of bio-ethanol (containing 5 ppm S) correlates with the type III Ni species and is influenced by both the Ni-content and the Ce/Zr ratio in the support. A catalyst of composition NiO (40 wt.%)-CeO2 (30 wt.%)-ZrO2 (30 wt.%) maintained its activity for more than 500 h without deactivation. (C) 2003 Elsevier Science B.V. All rights reserved.