Catalytic behaviors of the different metallic Ni species on Ni/Al2O3 catalyst surface were investigated in steam reforming of acetone and acetic acid to hydrogen. The different Ni species showed distinct activities for activation of acetic acid and acetone, gasification of coke precursors, and other secondary reactions such as methanation, water-gas shift reaction, CO disproportion and methane decomposition. The Ni species weakly interacted with alumina solely showed negligible reforming activity, while the Ni species mildly interacted with alumina was very active and efficient for steam reforming reaction. In comparison, the Ni species strongly interacted with alumina existed in bigger particle sizes, which resulted in the side reactions for by-product generation and consequently led to serious coking. Suppression of its presence on catalyst is the key point to obtain a highly selective and stable Ni/Al2O3 catalyst. (C) 2012 Elsevier B.V. All rights reserved.