A mathematical model has been developed to describe the isothermal operation of a single anode‐separator‐cathode unit cell in a redox‐flow battery and has been applied to the NASA iron/chromium system. The model, based on porous electrode theory, incorporates redox kinetics, mass transfer, and ohmic effects as well as the parasitic hydrogen reaction which occurs in the chromium electrode. A numerical parameter study was carried out to predict cell performance to aid in the rational design, scale‐up, and operation of the flow battery. The calculations demonstrate: (i) an optimum electrode thickness and electrolyte flow rate exist; (ii) the amount of hydrogen evolved and, hence, cycle faradaic efficiency, can be affected by cell geometry, flow rate, and charging procedure; (iii) countercurrent flow results in enhanced cell performance over cocurrent flow; and (iv) elevated temperature operation enhances cell performance.