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Silicon is a serious candidate to replace graphite in electrodes because it offers a specific capacity almost 10 times higher than that of carbonaceous materials. However, cycling performances of Si electrodes remain very limited because of the huge volume changes upon alloying and dealloying with lithium. A fine understanding of the lithiation mechanism of silicon electrodes will help to design more robust architectures. In this work, an amorphous silicon thin film has been used as a model for a better understanding of lithiation mechanism. Lithium distribution in the Si layer has been thoroughly investigated by coupling powerful characterization tools: X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (ToF-SIMS). In particular, cross-analysis of different lithiation states has been carried out. A lithiation front moving forward over the state of charge has been highlighted. The quantification of the LixSi alloy indicates a lithium amount much higher than that of the Li/Si ratio estimated in previous studies. This anomaly leads to a description of the lithiation mechanism based on the presence of fast diffusion paths for Li throughout the Si layer. These paths would be a second driving force for silicon alloying and lithium segregation at the collector interface. SEM observations of a FIB cut corroborate this mechanism.