Lithium phosphorus oxynitride (LiPON) is an amorphous solid lithium ion conductor commonly used in all solid state thin film batteries (TFBs) with LiCoO2 as cathode- and lithium as anode-material. TFBs exceed conventional Li ion batteries with respect to lifetime and safety but may suffer from high ion transfer resistances and interface reactions between the electrodes and the electrolyte. In this contribution we study interface layer formation between LiPON and metallic lithium using an in-situ X-ray photoemission spectroscopy (XPS) surface science approach. (C) 2014 Elsevier B.V. All rights reserved.