Deuterium absorption in Mg₇₀Al₃₀ thin films coated with a Pd layer and a Ta/Pd bilayer were investigated using in situ neutron reflectometry at room temperature and deuterium pressures up to 1.3 bar. The approach used provides a detailed profile, at the nanoscale, of the deuterium content inside the specific layers that constitute the films. It is found that Mg₇₀Al₃₀ can store up to 5 wt.% under these mild conditions following a two-step mechanism. The latter involves the deuteration of the top and bottom catalyst layers first, followed by the main Mg₇₀Al₃₀ layer. The presence of deuterium throughout the films in the early absorption stages evidences atomic deuterium spillover from the catalyst layers. The addition of a Ta layer between the Pd and Mg₇₀Al₃₀ was found to allow observable absorption at a pressure 10 times lower than on the Ta-free sample, without affecting the storage capacity. Our measurements imply that this improvement in kinetics is due to a lowering of the nucleation barrier for the formation of the hydride phase in the Mg₇₀Al₃₀ layer. ; peer reviewed: yes ; NRC Pub: yes ; system details: machine converted author identifier PE to PID, February 2012