Release of iron from various ferrisiderophores (ferripyoverdines, ferrioxamines B and E, ferricrocin, ferrichrome A, ferrienterobactin and its analog ferric N,N',N''-tri(1,3,5-Tris) 2,3-dihydroxybenzoylaminomethylbenzene) was obtained through an enzymic reduction of iron, involving NADH, FMN and the ferripyoverdine reductase of Pseudomonas aeruginosa PAO1. The iron released from the same complexes was also obtained through chemical reduction of iron involving FMNH2. Evidence is given that the enzymic process acts through a FMNH2 reduction; the P. aeruginosa enzyme, purified according to its ferripyoverdine-reductase activity [Hallé, F. & Meyer, J. M., Eur. J. Biochem. 209, 613-620], functions as a NADH:FMN oxidoreductase, the FMNH2 produced being able to chemically reduce the iron complexed by siderophores. The general occurrence of such a multi-step mechanism, which denies the existence of specific ferrisiderophore reductases, is discussed.