We report a newadditive, monobasic potassium arsenate (KH2AsO4), for improving the thermal stability of VFB catholytes. Using our standard accelerated testingmethodology at 30-70 degrees C, we showed that the effect increases continuously with increasing concentration of arsenate over the range investigated (0 - 0.10 mol dm(-3)). In comparison to similar experiments with phosphate (H3PO4), the magnitude of the effect was greater for arsenate. A combination of arsenate and phosphate was also effective. Based on these results, we speculate that other Group-V elements in the +5 oxidation state may also stabilize VFB catholytes. (C) The Author(s) 2019. Published by ECS.