Previous works showed that oxide-modified Al powder could react with water and generate hydrogen under ambient condition. The activation mechanism was believed to be the weakening of passive oxide film on Al particle surface due to all its constituents transforming into loose gamma-Al2O3 phase during the heat-treatment. In this work, a model experiment was performed, in which pure Al powder and the mixture of Al + gamma-Al2O3 powders were heat-treated in vacuum from room temperature to 600 degrees C. It was found that the induction time for the reaction of heat-treated pure Al powder with water decreased obviously when the heat-treating temperature was above 200 degrees C. However, the reaction induction time of the mixture of Al + gamma-Al2O3 powders was always considerably shorter than that of pure Al powder even if no heat-treatment was done. This implies that the catalytic effect of external oxides on the hydration of passive oxide film on Al particle surface plays a dominant role in promoting the Al-water reaction and hydrogen-generation. This clarification is also helpful in understanding the corrosion behavior of metal Al in different environment. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.