Water oxidation is a key reaction for the conversion of solar energy into chemical fuels, but effective water-oxidation catalysts are often based on rare and costly precious metals such as Pt, Ir or Ru. Developing strategies based on earth-abundant metals is important to explore critical aspects of this reaction, and to see whether different and more efficient applications are possible for energy systems. Herein, we present an approach to tuning a redox-active electrocatalyst based on the doping of molybdenum into the tungsten framework of [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–, known as the Weakley sandwich. The Mo-doped framework was confirmed by X-ray crystallography, electrospray ionization mass spectrometry and inductively coupled plasma optical emission spectrometry studies. The doping of molybdenum into the robust Weakley sandwich framework leads to the oxidation of water at a low onset potential, and with no catalyst degradation, whereby the overpotential of the oxygen evolution reaction is lowered by 188 mV compared with the pure tungsten framework.