A new octadentate terephthalamide desferrioxamine B (DFO) derivative, N-(2,3-dihydroxy-4-(methylamido)benzoyl)desferrioxamine B (DFOMTA), has been prepared. This ligand and three other DFO derivatives [N-(2,3-dihydroxybenzoyl)desferrioxamine B (DFOCAM), N-(2,3-dihydroxy-4-carboxybenzoyl)desferrioxamine B (DFOCAMC), and N-((1,2-dihydro-1-hydroxy-2-oxopyridin-6-yl)carbonyl)desferrioxamine B (DFOHOPO)] as well as their ferric complexes have been studied by potentiomeric, spectrophotomeric, and proton NMR titrations. All four DFO derivatives form six-coordinate 1:1 ferric complexes in aqueous solution, The catecholate derivatives are trishydroxamate complexes at low pH. At higher pH they form bishydroxamate-monocatecholate complexes by replacement of the terminal hydroxamate group with the catecholate group. In contrast, DFOHOPO forms a trishydroxamate complex at high pH and a bishydroxamate-monohydroxypyridinoate complex at low pH. These transformations parallel the relative acidity of the ligand functional groups : hydroxypyridinonate > hydroxamate > catecholate. The formation constants of the ferric complexes have been determined by spectrophotometric competition titrations (log K values) : DFOMTA, 34.8 (4); DFOCAM, 35.4 (4); DFOCAMC, 34.9 (2); DFOHOPO, 30.7 (3). Ail four derivatives are more powerful than their parent compound DFO as iron chelators at pH 7.4. The catecholate derivatives and their formation constants are the first determined for Fe(III) bound by two hydroxamate and one catecholate group and show UV/vis spectra characteristic of this coordination; these are good models for the pyoverdin siderophores.