Display omitted •The volume expansion of thin film amorphous silicon electrodes was investigated during potentiostatic lithiation.•Neutron reflectometry was used for an in-operando study.•A strongly non-linear correlation between volume and state-of-charge is found. Large volume modifications during electrochemical cycling of electrodes in Li-ion batteries often limit successful applications due to stress formation, electrode fracture and delamination from the current collector. In this study, we carried out investigations on the volume changes taking place during potentiostatic lithiation of the high capacity electrode material amorphous silicon. Thin film electrodes were investigated at potentials of 0.45, 0.28, 0.19 und 0.06 V vs Li/Li+ during lithiation using in-operando neutron reflectometry. We found a strongly non-linear correlation between volume and state-of-charge for each potential applied in strong contrast to the results of galvanostatic lithiation. A possible explanation might be that for high current densities occurring at the beginning of each potentiostatic lithiation step free volumes are created in the electrode material leading to disproportionate volume expansion.