Complexation of transition metals with strong organic ligands of unknown structure and origin plays an important role in their behavior and distributions in seawater. I present here a series of stability constants for complexes of several transition metals with the trihydroxamate siderophore desferrioxamine B (DFOB), representative of a class of small organic ligands that are exceptionally selective for Fe(III) and found in open ocean surface waters at low-picomolar concentrations. Stability constants for DFOB complexes with divalent metals are similar to conditional constants that have been measured in seawater for highly metal-specific, but unidentified organic ligands. Titration data indicate that free Hg2+ forms a very stable complex with DFOB, however the more toxic methylmercury cation does not. A revised scavenging model for yttrium and the rare earth elements suggests that their removal from seawater is dominated by sorption onto Mn oxides, yet not significantly influenced by biogenic silica or calcite.