The interplay between crystallization and phosphorus diffusion in the versatile synthesis of metal phosphide nanoparticles from well-defined metal nanoparticles is studied by using a favorable "P(0)" source for mechanistic studies: white phosphorus. In this study, the reaction of Ni, Fe, Pd, and Cu nanoparticles with P 4 was quantitative even at relatively low temperatures thanks to the high reactivity of this soluble "P" source. Intermediate amorphous alloys could be identified for the first time in the case of Fe and Pd, while the quantitative character of the reaction provided a selective and controlled access to Pd 5 P 4 versus PdP 2 and Cu 3 P versus CuP 2. Morphological evolution of the nanoparticles with temperature and M/P stoichiometry was also discussed and provided new insights in the kinetics of the reaction in each case. Hollow Ni 2 P and FeP nanoparticles were finally obtained while the particularly high stability of the amorphous plain Pd 3 P nanoparticles was uncovered.