Summary The structure of ferrioxamine H? a minor component of the sideramine complex of actinomycetes, was determined by spectroscopic investigations, degradation and synthesis. Ferrioxamine H is the Fe (111)-complex of 11, 22-dihydroxy-4, 12, 15, 23-tetraoxo-5, 11, 16, 22-tetraazatetracosanoic acid (2).