The Faradaic and non-Faradaic characteristics of a series of activated carbon particles (used to produce composite carbon–polymer electrodes for redox flow cells) have been determined using aqueous electrolytes (sulfuric acid and sodium polysulfide) at 295K. The particles were mounted as a circular section (ca. 0.80cm2) shallow packed bed of 2.5mm thickness in the direction of electrolyte flow (mean linear flow velocity≈6mms−1). Cyclic voltammetry in deaerated, 1moldm−3 H2SO4 at 295K indicated a specific capacitance in the range of 50–140Fg−1. Linear sweep voltammetry and galvanostatic step studies in an alkaline sodium polysulfide electrolyte (1.8moldm−3 Na2S2.11) have demonstrated marked differences amongst various types of activated carbon. Such differences are highlighted during galvanostatic charge–discharge cycling of half-cell electrodes in the polysulfide electrolyte. The electrochemical characteristics are compared to those based on (N2 adsorption) gas porosimetry measurements.