The iron-molybdenum cofactor of nitrogenase (FeMo-co) is synthesized in a multistep process catalysed by several Nif proteins and is finally inserted into a pre-synthesized apo-dinitrogenase to generate mature dinitrogenase protein. The NifEN complex serves as scaffold for some steps of this synthesis, while NifX belongs to a family of small proteins that bind either FeMo-co precursors or FeMo-co during cofactor synthesis. In this work, the binding of FeMo-co precursors and their transfer between purified Azotobacter vinelandii NifX and NifEN proteins was studied to shed light on the role of NifX on FeMo-co synthesis. Purified NifX binds NifB cofactor (NifB-co), a precursor to FeMo-co, with high affinity and is able to transfer it to the NifEN complex. In addition, NifEN and NifX exchange another [Fe-S] cluster that serves as a FeMo-co precursor, and we have designated it as the VK-cluster. In contrast to NifB-co, the VK-cluster is electronic paramagnetic resonance (EPR)-active in the reduced and the oxidized states. The NifX/VK-cluster complex is unable to support in vitro FeMo-co synthesis in the absence of NifEN because further processing of the VK-cluster into FeMo-co requires the simultaneous activities of NifEN and NifH. Our in vitro studies suggest that the role of NifX in vivo is to serve as transient reservoir of FeMo-co precursors and thus help control their flux during FeMo-co synthesis.