Electrospray mass spectrometry (ESMS) was used to determine the stoichiometry of a series of 1:1 iron(III) complexes with model dihydroxamic acids H(2)L, [CH3N(OH)C(O)](2)(CH2)(n), and with the natural dihydroxamate siderophore rhodotorulic acid, RA. Parent (or complex molecular ion) peaks were observed at m/z 314, 286, 258, and 230 for the model complexes where n = 8, 6, 4, 2, respectively, and at mit 398 for the analogous RA complex. The isotope distribution patterns in the ESMS strongly suggest that the model complexes exist in a dimeric form, Fe(2)L(2)(2+), when n = 2; 4. A monomeric structure, FeL(+), is consistent with the isotope distribution patterns for the ESMS of the model complex when n = 8, and for the RA complex. The model complex, n 6, exhibits more complicated ESMS data, suggesting that both monomer and dimer exist in solution. Molecular mechanics computational studies performed on the model complexes give a relative conformational strain energy, E(m), of 9, 10, 20, and 56 kcal/mol for n = 8, 6, 4, 2, respectively, when a monomeric structure is assumed. When a dimeric structure is assumed for the model complexes, the relative strain energy, E(d), is 14, 13, 13, and 12 kcal/mol. For the RA complex, relative strain energies are 10 and 16 kcal/mol for the monomeric and dimeric structures, respectively. The shift from monomer to dimer as n is decreased is discussed in terms of the increase in conformational strain enthalpy in the monomeric structure as n is decreased.