Carbon felts are commonly used as electrode materials for vanadium redox flow batteries (VRFBs). Their physical and chemical properties strongly influence the overall cell performance. The degradation of these materials due to parasitic side reactions (carbon corrosion, oxygen and hydrogen evolution) is partially responsible for performance losses of the cell [1]. Differential electrochemical spectrometry (DEMS) has been proven to be suitable for characterizing carbon materials, mainly in the context of fuel cell research [2–4]. Recently, Taylor et al. used DEMS to calculate the Faradaic efficiency of the V3+ reduction reaction on the oxidized edge and basal surfaces of graphite discs in a model study on the activity and stability of the negative electrode in VRFBs [5]. For our experiments, we modified a thin layer flow cell DEMS setup to conduct simultaneous electrochemical and online mass spectrometric measurements on commercial carbon felt electrodes. With this setup, half-cell tests, such as cyclic voltammetry, and coulometry of the redox couples under stopped flow of the electrolyte were performed. We used different potential windows, and two types of electrolytes while monitoring potential dependent H2, O2 and CO2 formation. At oxidizing potentials, we did not observe oxygen evolution, only carbon corrosion. In the presence of vanadium, an increase in CO2 and H2 formation at high respectively low potentials was observed. The DEMS method is capable to monitor side reactions occurring in VRFBs. Therefore, it is a useful tool to develop electrode materials, which exhibit a higher corrosion resistivity. 1. Eifert L, Banerjee R, Jusys Z, Zeis R (2018) Characterization of Carbon Felt Electrodes for Vanadium Redox Flow Batteries: Impact of Treatment Methods. J Electrochem Soc 165:A2577–A2586. doi: 10.1149/2.0531811jes 2. Colmenares LC, Wurth A, Jusys Z, Behm RJ (2009) Model study on the stability of carbon support materials under polymer electrolyte fuel cell cathode operation conditions. J Power Sources 190:14–24. doi: 10.1016/j.jpowsour.2009.01.078 3. Ashton SJ, Arenz M (2012) Comparative DEMS study on the electrochemical oxidation of carbon blacks. J Power Sources 217:392–399. doi: 10.1016/j.jpowsour.2012.06.015 4. Pérez-Rodríguez S, Sebastián D, Lázaro MJ, Pastor E (2017) Stability and catalytic properties of nanostructured carbons in electrochemical environments. J Catal 355:156–166. doi: 10.1016/j.jcat.2017.09.019 5. Taylor SM, Pătru A, Perego D, et al. (2018) Influence of Carbon Material Properties on Activity and Stability of the Negative Electrode in Vanadium Redox Flow Batteries: A Model Electrode Study. ACS Appl Energy Mater 1:acsaem.7b00273. doi: 10.1021/acsaem.7b002736. Eifert L, Banerjee R, Jusys Z, Zeis R (2018) Differential Electrochemical Mass Spectrometry of Carbon Felt Electrodes for Vanadium Redox Flow Batteries. ACS Appl Energy Mater 1 (12), 6714-6718 doi: 10.1021/acsaem.8b01550 Figure 1