The formation mechanism of platinum-copper (Pt-Cu) bimetallic clusters at the early stage within the water-in-oil microemulsion system of water/sodium bis(2-ethyl hexyl)sulfosuccinate/n-heptane is explored by in situ X-ray absorption spectroscopy. The reduction of platinum and copper ions and the formation of corresponding clusters are monitored as a function of the dosage of reducing agent, hydrazine (N2H4). Upon successive addition of the reducing agent, hydrazine (N2H4), five distinguishable steps are observed in the formation process of Pt-Cu clusters at the early stage. Both in situ X-ray absorption near-edge structure and extended X-ray absorption fine structure at the Pt L-III-edge suggest the Pt4+ --> Pt2+ --> Pt-0 reduction sequence, and analysis of the spectra recorded at the Cu K-edge confirms the Cu2+ --> Cu-0 reduction sequence. A corresponding structural model is proposed for each step to provide detailed insight into the mechanism of nucleation and growth of Pt-Cu bimetallic clusters.