IscA/SufA proteins belong to complex protein machineries which are involved in iron-sulfur cluster biosynthesis. They are defined as scaffold proteins from which preassembled clusters are transferred to target apoproteins. The experiments described here demonstrate that the transfer reaction proceeds in two observable steps: a first fast one leading to a protein-protein complex between the cluster donor (SufA/IscA) and the acceptor (biotin synthase), and a slow one consisting of cluster transfer leading to the apoform of the scaffold protein and the holoform of the target protein. Mutation of cysteines in the acceptor protein specifically inhibits the second step of the reaction, showing that these cysteines are involved in the cluster transfer mechanism but not in complex formation. No cluster transfer from IscA to IscU, another scaffold of the isc operon, could be observed, whereas IscU was shown to be an efficient cluster source for cluster assembly in IscA. Implications of these results are discussed.