The lithiation onset of amorphous silicon (a-silicon) films up to 10% state of charge (SOC) is characterized by a Li⁺-uptake region around 0.5 V vs a Li reference electrode. In the literature, this is commonly attributed to surface processes such as the formation of a solid electrolyte interphase layer and/or the reduction of the surface native oxide, and more seldom to bulk processes such as reduction of oxygen contaminations inside the silicon film and to silicon lithiation. This work presents evidence that this process is associated with the lithiation of elemental silicon using electrochemical and non-electrochemical measurements and including a discussion of literature data. Cyclic voltammetry performed on pre-lithiated a-silicon and a-silicon films with different oxygen concentrations and with different film thicknesses reveals a lithiation process that is not associated with the reduction of oxygen. Elemental depth-profiling with secondary ion mass spectrometry (SIMS) reveals a two-phase lithiation mechanism at the lithiation onset, which sharply delimits non-lithiated silicon from a Li-poor phase of constant Li content (Li_0.3Si). The published operando neutron reflectometry data suggest that this is also the case for oxygen-free Si single crystal wafers. SIMS measurements further show that the whole a-silicon thin film is converted into the Li-poor phase. This phase remains stable in the vicinity of the current collector even at 100% SOC, indicating that mechanical stress may play an important role.