We present a neutron reflectometry study of deuterium absorption in thin films of Al-containing Mg alloys capped with a Ta/Pd, Ni/Pd and Ti/Pd-catalyst bilayer. The measurements were performed at room temperature over the 0 1 bar pressure range under quasi-equilibrium conditions. The modeling of the measurements provided a nanoscale representation of the deuterium profile in the layers at different stages of the absorption process. The absorption mechanism observed was found to involve spillover of atomic deuterium from the catalyst layer to the Mg alloy phase, followed by the deuteration of the Mg alloy. Complete deuteration of the Mg alloy occurs in a pressure range between 100 and 500 mbar, dependent on the type of bilayer catalyst. The use of a Ti/Pd bilayer catalyst yielded the best results in terms of both storage density and kinetic properties. ; peer reviewed: yes ; NRC Pub: yes