Bromine complexing agents (BCA) are used to improve the safety of aqueous bromine electrolytes versus bromine outgassing in bromine electrolytes. In this work, cycling performance of hydrogen-bromine redox flow battery cells with 1-ethylpyridin-1-ium bromide ([C2Py]Br) as BCA in a bromine electrolyte with a theoretical capacity of 179.6 A h L−1 is investigated for the first time. The BCA leads to increased ohmic overvoltages. One cause of the ohmic drop can be attributed to [C2Py]+ cation interaction with the perfluorosulfonic acid (PFSA) membrane, which results in a drop of its conductivity. The BCA also interacts with bromine in the cell, by forming a non-aqueous fused salt second phase which exhibits a ten times lower conductivity compared to the aqueous electrolyte. A steep rise in cell voltage at the beginning of the charge curve followed by a regeneration of the cell voltage is attributed to this effect. Electrolyte crossover leads to an accumulation of [C2Py]+ in the electrolyte solution and intensifies both adverse processes. Under this condition only 30% of the theoretical electrolyte capacity of 179.6 A h L−1 is available under long term cycle conditions. However, electrolyte capacity is high enough to compete with other flow battery technologies