The mechanism of Pt and PtPb nanocrystal formation under supercritical ethanol conditions has been investigated by means of in situ X-ray total scattering and pair distribution function (PDF) analysis. The metal complex structures of two different platinum precursor solutions, chloroplatinic acid and Pt(acac)(2) (acac=acetylacetonate) provide atomic-scale detail about the nucleation mechanisms after initiation of the reaction with Pb(acac)(2) by heating. The stronger Pt-O chemical bonding in the Pt(acac)(2) precursor complex compared with the Pt-Cl bonding in the chloroplatinic acid precursor complex leads to a much slower reduction of the Pt center, and this allows more optimal co-reduction conditions providing a pathway for formation of phase-pure intermetallic PtPb product. The matching chemistry of the Pt(acac)(2) and Pb(acac)(2) precursors allow development of a facile continuous flow supercritical ethanol process for obtaining phase-pure hexagonal PtPb nanocrystals. The study thus highlights the importance of in situ studies in revealing atomic-scale information about nucleation mechanisms, which can be used in design of specific synthesis pathways, and the new continuous-flow process to obtain PtPb nanocrystals holds potential for large-scale production.