Complex formation of nickel(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (HL) and the aggregation of the monomer complex at the heptane-water interface have been kinetically studied by using centrifugal liquid membrane (CLM) spectrophotometry. The interfacial aggregation of ligand in NiL2 complex was observed as the growth of a remarkably intense and narrow absorption band (J-band) at 588 nm, showing a bathochromic shift from the absorption maximum of the monomer complex (569 nm), accompanied by a decrease of the absorbance of the free ligand at 452 nm. The formation of aggregate was initiated when the interfacial concentration of the monomer complex attained the critical aggregation concentration (cac) of 2.4 × 10-10 mol cm-2. The initial formation rate of the interfacial aggregate was proportional to concentrations of both free Ni(II) and free ligand after the attainment of cac. The observed rate constant for the aggregation, 4.1 × 101 M-1 s-1, was smaller than the formation rate constant of NiL2 complex at the interface. A long period observation of the profile of absorbance change showed an oscillation of the interfacial aggregate concentration until all of the ligand was consumed, suggesting a subsequent flocculation of the aggregate in the rotating cell.