An operating intercalation battery system was prepared in ultrahigh vacuum and investigated by concurrent photoelectron spectroscopy (PES) and electrochemical methods. A TiS2 film was deposited on a Na-beta "-Al2O3 plate, and a graphite layer was placed on the back side of the plate and used as a counter electrode. The voltage between the TiS2 and the graphite was controlled directly at the PES measurement position. Upon lowering the applied voltage, the amount of Na increased linearly with increasing charge and changes on the substrate PES signals were observed, which are in agreement with previous results found with Na deposition. Electrons were transferred to the host Ti 3d states only up to a Na/Ti ratio of approximate to 0.6. The resulting shift of the Fermi level led to a related binding energy shift of all emission lines along the entire composition range.