Abstract Electrodes made of amorphous silicon are a promising alternative to graphite as anode material for the next generation of high-capacity Li-ion batteries. In order to optimize such batteries the mechanism of lithium incorporation into the electrode during charging has to be elucidated. In the present study we measured the modification of lithium distribution taking place during galvanostatic lithiation of about 600 nm thick amorphous silicon film electrodes at low current densities of about 30 μA/cm² (∼ C/14) by Secondary Ion Mass Spectrometry. The results were confirmed by X-ray Photoelectron Spectroscopy. The results indicate a two-step lithiation procedure where the electrode is transformed first into a homogeneously lithiated Li _x Si phase with a low Li content of x ≈ 0.3. During later stages of the lithiation process, the results are in agreement with the penetration of a highly lithiated phase via a moving phase boundary, as also observed for crystalline silicon.