Structures composed of transition metal oxides can display a rich variety of electronic and magnetic properties including superconductivity, multiferroic behavior, and colossal magnetoresistance.[1] An additional property of technological relevance is the bipolar resistance switching phenomenon [2–4] seen in many perovskites [5–7] and binary oxides [8] when arranged in metal/insulator/metal (MIM) structures. These devices exhibit electrically driven switching of the resistance by 1000x or greater and have recently been identified [9] as memristive systems, the fourth fundamental passive circuit element.[10, 11] A full understanding of the atomic-scale mechanism and identification of the material changes within the oxide remains an important goal.[12] Here, we probe within a functioning TiO 2 memristor using synchrotron-based x-ray absorption spectromicroscopy and transmission electron microscopy (TEM). We observed that electroforming of the device generated an ordered Ti 4O 7 Magnéli phase within the initially deposited TiO 2 matrix. In a memristive system,[11] the flow of charge dynamically changes the material conductivity, which is “remembered” even with the removal of bias. While bipolar resistance switching of metal oxides has been observed since the 1960s,[2, 4] only recently has the connection to the analytical theory of the memristor been made.[9] In an attempt to describe microscopically the source of the resistance change, many physical models have been put forth, including generation and dissolution of conductive channels,[3, 6] electronic trapping and space-charge current limiting effects,[13] strongly correlated electron effects such as a metal-insulator transition,[14] and changes localized to the interface.[15] Identifying the correct model and quantifying its physical parameters has been difficult using primarily electrical characterization. Meanwhile, direct physical characterization [7] requires the capability of observing subtle material changes, such as vacancy creation, occurring in a nanoscale volume buried between two metal contacts. Recently, Kwon, et al.[16] performed cross-sectional TEM studies of the unipolar resistive switching of TiO 2, revealing the presence of nanoscale Magnéli phase conductive channels following device operation and extraction of a region of interest. A Ti 4O 7 phase was confirmed from the temperature dependence of the conductance. Concurrently, we investigated the bipolar mode of TiO 2 switching, using non-destructive spatially-resolved x-ray absorption and electron diffraction to report on the associated chemical and structural changes of a functioning device. To enable the transmission measurements, a vertical MIM crosspoint device was fabricated on a thin free-standing Si 3N 4 window, as illustrated in Figure 1a. The device stack consisted of Cr (5)/Pt (15)/TiO 2 (30)/Pt (30 nm) grown on a Si/Si 3N 4 (20 nm) substrate. The device consisted of a 2 µ m wide bottom electrode and a 3 µ m wide top electrode patterned perpendicular to each other with their overlap defining the active junction area. The junction was centered within the 60× 60 µ m 2 free-standing window (Figure 1b). The TiO 2 switching layer, which extends across the entire substrate, was sputter deposited from a titania source onto the substrate held at 250 C. Following an initial electroforming step, the device exhibited reversible bipolar resistance switching (Figure 1c) between a low resistance of 20 k Ω (ON) and a high resistance of 1 M Ω (OFF), as measured at low bias. One issue of concern was whether devices fabricated on thin windows would behave similar to devices on thicker substrates; strain effects and the inability to sink the …