This work explores a novel reference cell for simultaneously assessing the compositions of the positive and negative electrolytes in a vanadium redox flow battery. The reference cell separately measures the potentials of the positive and negative electrolyte streams supplying a flow battery with respect to stable hydrogen electrodes. The compositions of the two streams are determined from the individual electrode potentials. The reference cell contains three distinct sub-cells connected by a single contiguous membrane: 1) a vanadium redox cell that continuously measures the open-circuit potential of the battery, 2) a compartment designed to leach vanadium ions from the membrane, and 3) a hydrogen cell that provides the individual potentials of the two electrode streams versus a stable reference. Compositional information determined using the reference cell leads to accurate predictions of the capacity of a connected working cell during galvanostatic cycling. (C) 2019 The Electrochemical Society.