Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH4 is the only complex metal hydride identified so far that combines favorable thermodynamics with a reasonable hydrogen storage capacity (5.5 wt %) when decomposing in two steps to NaH, Al, and H2. The slow kinetics and poor reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. In an alternative approach we studied the influence of a reduced NaAlH4 particle size and the presence of a carbon support. We focused on NaAlH4/porous carbon nanocomposites prepared by melt infiltration. The NaAlH4 was confined in the mainly 2−3 nm pores of the carbon, resulting in a lack of long-range order in the NaAlH4 structure. The hydrogen release profile was modified by contact with the carbon; even for ∼10 nm NaAlH4 on a nonporous carbon material the decomposition of NaAlH4 to NaH, Al, and H2 now led to hydrogen release in a single step. This was a kinetic effect, with the temperature at which the ...