The reduction of the ammonium nickel phosphate NiNH4PO4 center dot H2O precursor into nickel phosphide (Ni2P), a highly active phase in hydrotreating catalysis, was studied using a combination of magnetic susceptibility and in situ X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. The transformation of NiNH4PO4 center dot H2O into Ni2P could be divided into three distinguishable zones: (1) from room temperature to 250 degrees C, the NiNH4PO4 center dot H2O structure was essentially retained; (2) from 300 to 500 degrees C, only an amorphous phase was observed; (3) above 500 degrees C, a crystallization process occurred with the formation of Ni2P. An in situ XAS study and magnetic susceptibility measurements clearly revealed for the first time that the amorphous region corresponds to the nickel pyrophosphate phase alpha-Ni2P2O7. The phosphate reduction into phosphide did not start before 550 degrees C and led to the selective formation of Ni2P at 650 degrees C,