The flavoprotein component (SiR-FP) of the sulfite reductase from Escherichia coli is an octamer containing one FAD and one FMN per polypeptide chain. SiR-FP60, a SiR-FP fragment starting with alanine-52, was overexpressed in E. coli and purified as a monomer. The N-terminal part of the native protein contains thus all the determinants required for the polymerization. SiR-FP60 retains both FAD and FMN with comparable contributions of the two flavins and the catalytic properties of SiR-FP. Thus, SiR-FP60 can be considered as a reliable simplified model of the sulfite reductase flavoprotein component. The formation and the stabilization of the neutral FMN semiquinone is thermodynamically favorable in SiR-FP60 upon reduction with photoreduced deazaflavin, dithionite, or NADPH. Generation of FMNH* is explained from a disproportionation of electrons between the reduced and oxidized FMN moieties during an intermolecular reaction, as shown with SiR-FP23, the FMN-binding domain of SiR-FP. The neutral FAD semiquinone can be observed only within SiR-FP43, the isolated FAD-binding domain. NADPH was used as a titrant or in excess to demonstrate that electron transfer is possible only because the FMN cofactor is coupled to FAD as an electron acceptor in the protein. The electron distribution within the various reduced forms of SiR-FP60 has been compared with that of the reduced forms of cytochrome P450 reductase, bacterial cytochrome P450, and nitric-oxide synthase. Despite the conservation of the bi-flavin-domain structure between these proteins over evolutionary time, each of them provides significantly different flavin reactivities.