Bimetallic phosphide nanoparticles are drawing a great interest as a family of nanomaterials. Controlling their fine features such as surface composition, core crystal structure and overall composition is a key to further application. Copper and nickel are particularly interesting first-raw metals for their abundance and relevance in several branches of catalysis. In order to synthesize crystalline bimetallic phosphide Ni-CuP nanoparticles, core-shell copper-nickel nanoparticles were reacted with white phosphorus (P 4). Surprisingly, hollow monocrystalline (Ni,Cu) 2 P nanoparticles were formed alongside Cu nanoparticles and crystallized in a phase isostructural to Ni 2 P. Using a combination of local and ensemble analytic techniques, we showed that this unique structure is the result of several competing processes: phosphorus migration, interaction of stabilizing ligands with copper as well as metal phosphide phase crystallization. This study provides important mechanistic insights to rationalize bimetallic phosphide nanoparticles syntheses. Beyond metal phosphides, this well-characterized case study about competing diffusion and crystallization processes is of major relevance for the advancement of materials sciences at the nanoscale.