This work focuses on the performance and stability of selected commercial carbon electrode materials before and after heat-treatment in an operating all-vanadium redox flow battery (VRB). Heat treatment results in improved cell performance for all tested materials, with SGL 39 AA carbon papers and SIGRACELL GFD4.6 EA carbon felt showing the best performance. Further investigation of these two materials by in situ reference electrode measurements reveal improvements after heat-treatment that originate mainly from the negative electrode or V2+/V3+ side of the cell. Upon extended cycling, carbon felt is found to be stable. Carbon papers however, show significant performance losses originating from the negative electrode side. The potential limit during charging and the exposure to very negative potentials appears to be a critical issue at the negative electrode in the VRB. Analysis of both materials after cycling by scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy reveal significant differences in their surface chemistry, structure and morphology. These differences give valuable insights into the behavior and degradation of different carbon materials used in VRBs. (C) The Author(s) 2017. Published by ECS. All rights reserved.