Synchrotron-based time-resolved X-ray diffraction was used to study in situ the crystalline phases present during the preparation of bulk and silica-supported MoP, Ni2P, and MoNiP by reduction of oxidic precursors in hydrogen. Independent of the type of oxidic precursor used or the presence of silica as a support, the formation of the metal phosphides occurs at temperatures between 600 and 800 degreesC. Since the common species in all the cases are phosphate-type groups (POx), it seems that their reduction by hydrogen is the final and determining step in the formation of MoP, Ni2P, and MoNiP. Silica-stabilized phosphide phases were detected during the synthesis of Ni2P/SiO2 and MoP/SiO2 catalysts. In the case of Ni2P/SiO2, before the appearance of the final phosphide, strong diffraction lines are observed for Ni12P5. First-principles density functional calculations for bulk MoP, Ni2P, and MoNiP indicate that the Ni-P and Mo-P bonds in these compounds have a small degree of ionic character. For MoP, Ni2P, and MoS2 a correlation is found between the electronic properties of the metal cations and their hydrodesulfurization activities when supported on silica. Surprisingly, a MoNiP/SiO2 catalyst is much less active than either MoP/SiO2 or Ni2P/SiO2 catalysts.