The excellent bifunctional catalytic activity of nickel phosphide (Ni2P) for water splitting is reported. Ni2P, an active hydrogen evolving catalyst, is shown to be highly active for oxygen evolution. Only 290 mV of overpotential is required to generate a current density of 10 mA cm(-2) in 1 M KOH. Under oxygen evolving conditions, Ni2P undergoes structural modification to form a Ni2P/NiOx core-shell assembly, the catalytic active species. Ni2P is applied on both electrodes of an alkaline electrolyser and a current density of 10 mA cm(-2) is generated at 1.63 V.