Progressing from graphite to silicon-based anodes for lithium-ion batteries increases the importance of a depth-resolved understanding of the reversible and irreversible processes across the thickness of the anode electrode. Considerable changes in electrode volume and mass loading upon (de-)lithiation make silicon electrodes more susceptible to continuous side reactions and to the isolation of active material particles, leading to non-uniform and accelerated electrode degradation. Here, we investigate the evolution of lithium concentration profiles across the thickness of porous silicon-graphite (SiG) electrodes (similar to 20 mu m thickness, similar to 1.7 mAh cm(-2)) with 35 wt% silicon nanoparticles during the first (de-)lithiation cycle. Using ex situ neutron depth profiling (NDP), we monitor depth-and quantity-resolved (i) the solid-electrolyte-interphase (SEI) formation, (ii) the (de-)lithiation of the active materials, as well as (iii) the changes in the total lithium content as a function of the state-of-charge (SOC) and depth-of-discharge (DOD). The results provide depth-resolved information about reversible and irreversible processes occurring during the formation of SiG electrodes, and thus offer insight into the formation process of silicon-based electrodes. (C) The Author(s) 2019. Published by ECS.