Mg-doped WO3 with band gap energy of about 2.6 eV is a viable photocatalyst for visible light-induced water splitting in the presence of hole scavengers. The conduction band edge position of p-type Mg-doped WO3 was -2.7 V versus SCE at pH 12. By doping Mg on WO3 the conduction and valence band positions were shifted by 2.25 V negatively, leading to a conduction band edge position which was negative enough for H+ ions to be reduced thermodynamically, with little change in band gap energy. The negative shift in band position might be ascribed to lowering of the effective electron affinity of WO3 by doping Mg with a very low electron affinity.