Activation of coagulation factor X (fX) by activated factors IX (fIXa) and VIII (fVIIIa) requires the assembly of the enzyme–cofactor–substrate fIXa– fVIIIa–fX complex on negatively charged phospholipid membranes. Using flow cytometry, we explored formation of the intermediate membrane-bound binary complexes of fIXa, fVIIIa, and fX.