Previous experiments showed that gamma-Al2O3-modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH- ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH- ions react with Al and release H-2. Because of the limited H-soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H+ species, H-2 molecules accumulate and form small H-2 gas bubbles at the Al:Al2O3 interface. When the reaction equilibrium pressure in H-2 bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H-2 bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as-received Al powder could continuously react with water at temperatures above 40 degrees C and under low vacuum, because the vacuum makes the critical gas pressure in H-2 bubbles decrease as well.