Carbon fiber-based fabrics such as felts, cloths or papers are porous electrode materials that are widely used in redox flow batteries. This study investigates the effect of the carbon fiber properties on hydrogen evolution at the negative electrode, which occurs as an important side reaction in redox flow batteries employing acidic electrolytes. The lowest hydrogen evolution rate was observed at carbon fibers with a high content of graphitic domains which have been subjected to surface oxidation pre-treatment. By contrast, felt materials consisting of fibers with a largely amorphous character produce elevated amounts of hydrogen during battery charging. Differences between rayon- and polyacrylonitrile (PAN)-derived carbon fibers point toward the important role of nitrogen species.