The devices used in this study were 500×500 nm2 crossbar structures with 20 nm thick TiN electrodes and 100 nm of reactively sputtered amorphous TaOx. The devices were electroformed and electroformed/switched with the current compliance of 10-20 μA. The data have been gathered in several imaging modes of electron microscopy including bright field, High Angle Annular Dark Field (HAADF), Energy Dispersive X-ray Spectroscopy, and low loss and core regions of Electron Energy Loss Spectroscopy (EELS). The entire volume of the functional layer remained amorphous with no signs of crystallization. It contained only one area of bright contrast in HAADF images corresponding to the conductive filament with the diameter of approximately 50 nm. The maps produced by all techniques clearly indicate an increase of [Ta]/[O] ratio within the filament. In addition to overall change of the composition, the core of the filament shows a fine structure of the composition on the scale of 5-10 nm. We interpret the changes in distribution as due to thermodiffusion in the temperature gradient induced by current constriction when the device enters the negative differential resistance region of its I-V. The temperature distribution during this process has been estimated using a self-consistent finite element model.