a Polymeric carbon nitride (g-C 3 N 4 ) films were synthesized on polycrystalline semiconductor CuInS 2 chalcopyrite thin film electrodes by thermal polycondensation and were investigated as photocathodes for the hydrogen evolution reaction (HER) under photoelectrochemical conditions. The composite photocathode materials were compared to g-C 3 N 4 powders and were characterized with grazing incidence X-ray diffraction and X-ray photoemission spectroscopy as well as Fourier transform infrared and Raman spectroscopies. Surface modification of polycrystalline CuInS 2 semiconducting thin films with photocatalytically active g-C 3 N 4 films revealed structural and chemical properties corresponding to the properties of g-C 3 N 4 powders. The g-C 3 N 4 /CuInS 2 composite photocathode material generates a cathodic photocurrent at potentials up to +0.36 V vs. RHE in 0.1 M H 2 SO 4 aqueous solution (pH 1), which corresponds to a +0.15 V higher onset potential of cathodic photocurrent than the unmodified CuInS 2 semiconducting thin film photocathodes. The cathodic photocurrent for the modified composite photocathode materials was reduced by almost 60% at the hydrogen redox potential. However, the photocurrent generated from the g-C 3 N 4 /CuInS 2 composite electrode was stable for 22 h. Therefore, the presence of the polymeric g-C 3 N 4 films composed of a network of nanoporous crystallites strongly protects the CuInS 2 semiconducting substrate from degradation and photocorrosion under acidic conditions. Conversion of visible light to hydrogen by photoelectrochemical water splitting can thus be successfully achieved by g-C 3 N 4 films synthesized on polycrystalline CuInS 2 chalcopyrite electrodes.