The operation of a gas-phase Br2–H2 flow battery is analyzed via a mathematical model and compared to experimental data. The model predicts the operating conditions of the cell in both fuel-cell (i.e., discharge) and electrolysis (i.e., charge) mode as a function of current, inlet gas composition, flow rate, and pressure differential across the membrane. The analysis reveals that gas-phase Br2/HBr reactants significantly enhance mass transfer, which enables higher currents densities to be achieved compared to a liquid-fed system. A key feature of the model is water transport across the membrane, which determines membrane conductivity, reactant concentration and undesired condensation. The model is used to provide insight into cell operation, including operating conditions needed to avoid water condensation.