To gain new insights into the formation of the solid electrolyte interphase (SEI) as a basis for the safe and efficient use of new anode materials, SEI formation on silicon and lithium titanate (LTO) anodes was studied by electrochemical impedance spectroscopy (EIS) and ex situ X-ray photoelectron spectroscopy (XPS) measurements. EIS measurements performed at equidistant voltage intervals provided insights into SEI formation; ex situ XPS measurements supplied data on the chemical composition of the SEI layer. On the silicon anodes, a reduction of the resistance in the second cycle was observed, which suggests the formation of a stable SEI with SiO2, Li4SiO4, LiF, and different carbonates as its main components. On the LTO anodes, however, the resistance increased by a factor of two, indicating incomplete SEI formation. Here, LiF and different carbonates were identified as the SEI's main components.