Intermetallic nanoparticles (NPs), defined by an atomically ordered structure with high stability, have shown enhanced catalytic properties as compared to their disordered alloy counterparts.^1,2 The enhancement in catalytic properties is partially attributed to changes in the nature of available surface sites, which are induced by changes in the crystal structure.^3,4, To advance green energy solutions and pave the way for new improved catalytic materials comprised of intermetallic NPs, it is therefore crucial that we understand what controls the formation of intermetallic NPs to synthetically promote these structures. By carefully selecting the additives used in a PdCu NP synthesis, we show that monodisperse, intermetallic PdCu NPs can be synthesized in a controllable manner. In this synthesis, the additives iron(III) chloride and ascorbic acid are used to control both the morphology and polymorph of the intermetallic NPs. Ascorbic acid provides a fast reduction of the ionic metal precursor species, while iron(III) chloride facilitates ligand exchange with the metal ion complexes and can assist in oxidative etching. Combined, ascorbic acid and iron(III) chloride provide a synergetic effect resulting in precursor reduction and defect-free growth; ultimately leading to monodisperse intermetallic PdCu NPs. Using in situ X-ray total scattering and pair distribution function analysis, we follow the disorder-order transformation all the way from the initial precursor structure to the formation of intermetallic PdCu NPs. We report a hitherto unknown transformation pathway that diverges from the commonly reported co-reduction disorder-order transformation. A Cu-rich structure initially forms, followed by incorporation of Pd(0) into the structure to obtain the disordered alloy PdCu structure. The formation of a Cu-rich structure suggests that it is not essential that the metallic species reduce at the same rate to ultimately form stoichiometric intermetallic PdCu NPs, as previously believed. These findings underpin the importance and strength of performing further combined multi-technique studies to uncover the driving force of intermetallic NP formation. When we understand how intermetallic NPs are formed, these mechanistic insights might open new opportunities to expand our library of intermetallic NPs by exploiting synthesis by design.