In oceanic systems, microorganisms secrete siderophores to facilitate the uptake of Fe(III) which is a scarce and essential nutrient. Siderophores are capable of binding to metal ions other than Fe3+. In this research, desferrioxamine B was used as a representative siderophore and the stability constants for its complexation with Cu2+ and Ni2+ were determined at seawater ionic strength (0.7 M). Stability constants, log β1, log β2, and log β3 for the Ni-DFOB complex were found to be 4.65±0.04, 7.72±0.03, and 9.76±0.07, respectively. Stability constants for Cu(II), log β1 and log β2, were determined to be 7.86±0.06 and 13.09±0.04, respectively, but log β3 could not be resolved from titration data in the presence of excess DFOB, indicating that Cu2+ does not form a bond with the third hydroxamate group of the siderophore.