Oxygen evolution was studied on semiconducting IrxRu1-xS2 single crystals. Degeneracy was present for all iridium-containing compounds. The catalytic current density corrected for surface roughness, within experimental error, was not strongly dependent on the iridium content. The Tafel slope, however, showed a positive linear variation between 180 and 300 mV dec(-1) with increasing iridium content (0.5-50%). This catalytic behaviour already observed on mixed Ru-Ir-oxide electrodes, with a Tafel slope by a factor of 5 lower, suggests that the catalytic sites on our semiconducting electrodes possess an oxide-like nature. Differential electrochemical mass spectroscopy (DEMS) measurements have also revealed a higher Tafel slope and that oxygen evolves at an overpotential of 0.2 V, comparable to oxide electrodes. The higher Tafel slopes observed on IrxRu1-xS2 electrodes are discussed with respect to their semiconducting properties. Hydrogen evolution was additionally investigated.