Manganite-based devices have shown promising resistive switching properties, the performance of which strongly depends on the electrodes used to build them. Their nature affects the physical properties of the metal/manganite interfaces, leading to different resistive switching responses and mechanisms, which are difficult to fully understand using conventional ex situ characterization techniques. Herein, the switching mechanisms taking place in LaMnO 3+δ-based memristive devices with two different noble metal (Au and Pd) top electrodes have been studied using operando high-energy X-ray photoelectron spectroscopy (HAXPES). The difference between HAXPES spectra obtained for devices in different resistance states has allowed the elucidation of the redox mechanism taking place in the devices. The HAXPES spectra indicate a reduction (oxidation) of the interfacial LMO when switching from high to low (from low to high) resistance states. The presence of an inert electrode instead of an oxidizable one results in oxygen being released out of the lattice instead of reacting with the electrode. In addition, HAXPES energy shifts induced by the electric field have been used to probe the resistance drop inside the device at different positions and for different resistance states.