A series of supported Ni/gamma-Al2O3 samples were prepared and then used to identify the surface Ni species. The samples were characterized by ICP-AES, XRD, TPR, and XPS. The results indicated that two types of Ni species occurred on the samples with Ni loading < 9.1 wt%, while four types of Ni species occurred on the samples with Ni loading > 9.1 wt%. The reduction temperatures around 350, 500-520, 620-650, and 750-800 degrees C corresponding to Ni species were related to amorphous NiO, crystal NiO, NiAl2O4, and NiAlxOy (x > 2), respectively. A correlation was made between the catalytic activities and the surface Ni species. For the catalysts that were difficult to reduce (Ni loading < 9.1 wt%), the activities were very poor at low reduction temperatures: for the easily reducible catalysts (Ni loading > 9.1 wt%), the activities were excellent in a wider reduction temperature range from 400 to 800 degrees C. (C) 2009 Elsevier B.V. All rights reserved.