A combined modelling-experiments approach is presented with the aim of determining in-situ key characteristics of a RFB. The porous electrode impedance was calculated and measured without and with a faradaic reaction, by means of a symmetric cell. The technique was applied to the specific [Fe(CN)6]3-/[Fe(CN)6]4- couple whose interest for RFB application is substantial. The interpretation of Electrochemical Impedance Spectroscopy (EIS) data enabled the determination of the key membrane and electrode resistances and the investigation of their evolutions during the battery operation. The effects of the cation composition in the supporting electrolyte, the carbon material and the cycling were analysed. The results gave insights on degradation mechanisms that can hinder RFB performance, and delineated the phenomena occurring during electrolyte circulation or during battery operation.