In the ongoing search for new negative electrode materials for lithium-ion batteries, amorphous silicon with a theoretical specific capacity of almost 4000 mA h g−1 is still one of the most promising candidates. In order to optimize cycling behavior, prelithiation of silicon is discussed as possible solution. Yet, little is known about kinetics in the Li-Si system, especially with a low lithium content. Using neutron reflectometry as a tool, lithium permeation through amorphous LixSi layers was probed during annealing. From the results a lithium permeability (diffusivity×solubility) of P=(3.3±0.9)×10−21 m2 s−1 is derived for LixSi (x≈0.1), which is identical to that of pure amorphous silicon.