Spectrophotometric measurements of the reaction of ferrioxamine B (FeHDFB+) with 1,10-phenanthroline (phen) reveal the presence of a ternary intermediate complex in both aqueous solution and an aqueous solution of 0.16 M sodium dodecyl sulfate (SDS). The stoichiometry of the intermediate is Fe(H2DFB)(phen)(2+) on the basis of a Schwarzenbach analysis of spectrophotometric data obtained at variable pH and phen concentrations. The ternary complex formation constant for the reaction FeHDFB+ + H+ + phen reversible arrow Fe(H2DFB)(phen)(2+) is log K = 6.96 in aqueous solution and log K = 8.64 in aqueous 0.16 M SDS. The enhanced stability of Fe(H2DFB)(phen)(2+) in micellar solution was analyzed in terms of the pseudophase ion-exchange (PPIE) model of micellar reactions. The association constants for the binding of each reactant to the micellar pseudophase were measured by ultrafiltration. According to PPIE model calculations, the enhanced stability of Fe(H2DFB)(phen)(2+) in micellar SDS arises from a proximity effect created by the high local concentrations of reactants in the micellar pseudophase. The calculations also indicate that an inhibitory medium or compartmentalization effect is operative since the observed micellar enhancement is much smaller than predicted by the PPIE model. The micellar stabilization of the Fe(H2DFB)(phen)(2+) intermediate and the overall conversion of FeHDFB+ to Fe(phen)(3)(2+) are discussed as a possible model system for siderophore iron release in microbial organisms.