Three siderophores mirubactins B–D (4–6) were identified as the degradation products of previously isolated mirubactin (1). Their structures were revealed by HR-ESI-MS/MS, NMR analyses, and density functional calculations, among which 4 contains an unusual cyclic amidine functionality. Cyclic voltammetry showed that 5 and 6 have reduced iron complexing capacity. Mirubactin (1) and Fe(III) could also form a stable complex, which may be an ingenious approach to compete for iron acquisition by the producing organisms.