Monitoring the state-of-charge (SOC) in redox flow batteries is indispensable as a diagnosis tool to detect changes in the electrolyte concentration that can deteriorate the battery performance. Existing methods, which measure electrical variables of the cell or are dependent on recalibration during battery operation, become time consuming for practical uses. Here we present a simple and absolute method for monitoring the SOC in iron aqueous redox flow batteries. The method is based on the determination of the Fe(II) concentration with photometry after forming the complex of Fe(II)-1,10-phenanthroline. It can be applied in both the negative and positive electrolytes, and it is independent of the pH, concentration, and temperature of the electrolytes. The SOC values are compared with the redox potential of the negative electrolyte during chronopotentiometry experiments using different cells in a ferro/ferricyanide (positive electrolyte) and Fe(III)/(II)-triethanolamine (negative electrolyte) system. The SOC values as a function of the redox potential measurements follow a Nernst equation, which demonstrates the validity of the method. (C) The Author(s) 2018. Published by ECS.