The stability and the electrocatalytic activity of polycrystalline tungsten monocarbide (WC) and platinum (Pt) electrodes in a concentrated polysulfide electrolyte were examined in an electrochemical half-cell and an integrated photoelectrochemical cell (PEC) device. Cyclic voltammetry and linear sweep voltammetry measurements found WC electrodes to be approximately twice as active as Pt electrodes toward polysulfide reduction. Electrochemical and X-ray photoelectron spectroscopy (XPS) measurements indicated that WC was stable in a polysulfide electrolyte, with no appreciable W sulfide formation. In contrast, XPS showed significant levels of sulfide species on the Pt surface, which might explain the differences in the electrochemical behavior of the two electrode materials. The performance of integrated PEC devices comprised of thin-film CdSe photoanodes and WC or Pt counter electrodes in a polysulfide electrolyte was consistent with that predicted from electrochemical half-cell measurements.