Nanoscale structural cavities in ionomer membrane films were used as templates for the facile synthesis of small aluminum nanoparticles via catalytic decomposition of an alane precursor. The loading of reactive aluminum in the composite film could be varied, up to more than half of the film weight. While the embedded nanoparticles were protected by the membrane structure from any significant oxidation for the composite films to exhibit surprising stability in ambient air, they could be fully accessed in base water for the hydrogen production quantitatively. The templated synthesis may represent a new route for stable aluminum nanoparticles and related energetic nanomaterials.