Model-type thin film all-solid-state batteries (ASSB) of the type Li/”LiPON”/Li4Ti5O12 are characterized by impedance spectroscopy as function of the state of charge. Both the contact of Li4Ti5O12 (LTO) to the substrate and current collector as well as the Li ion transport through the LTO thin film prove to be important factors for the rate limitation of the cell. The ionic charge transfer resistance between Li4Ti5O12 and “LiPON” is proven to not be a rate limiting factor. The impedance measurements are interpreted in terms of the mechanism of phase change in the Li4Ti5O12 thin film and the data indicate a solid solution mechanism of phase change. The resulting data are compared to recent results for the Li/”LiPON”/LiCoO2 system, and several key similarities and differences are highlighted mostly involving the solid electrolyte/Li interface and the degradation thereof. The results are meant to improve the understanding of interfaces in ASSB in general rather than to advance high performance batteries.