The complexation mechanism and aggregate formation of his [2-(5-bromo-2-pyridylazo)-5-diethylaminophenolate] iron(II) and iron(III) complexes at the heptane-water interface were studied spectrophotometrically by the high-speed stirring method and the centrifugal liquid membrane method. Furthermore, the reduction process of the Fe(III) complex with ascorbic acid at the interface was spectrophotometrically observed. The chemical compositions of the interfacial aggregate of complexes have been proved by the X-ray photoelectron spectroscopy. The aggregation of the complex at the interface was observed as a red-shifted, very strong and narrower absorption band with respect to the absorption band of the monomer complex. The aggregate of Fe(111) complex showed more shifted spectrum than that of Fe(II) complex, which proposed the larger aggregation number of Fe(III) aggregate (n = 8) than that of Fe(II) aggregate (n = 3). The obtained rate constants of interfacial aggregation were smaller than rate constants of interfacial monomer complexation, because the formation of aggregate required the assembly of the monomers. (C) 2004 Elsevier Inc. All rights reserved.