The interfacial activity of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HL) and its complexation kinetics with Ni(II) in the chloroform-water phase pair have been studied. The interfacial activities are in the order NiL2 > L- > HL. The rate limiting step in the complexation of Ni(II) by L- is the formation of NiL+, and the interfacial rate constant is 4 times larger than the bulk value. The rate limiting step in the back-extraction (dissociation) of NiL2 is the reaction of NiL+ with proton, and the bulk and interfacial rate constants for this reaction are essentially the same. The dissociation reaction in neutral micelles formed by the surfactant Brij 35 occurs mainly at the aqueous-micellar interface with a rate constant that is similar to that at the CHCl3-H2O interface. The band widths of Ni(II) in centrifugal partition chromatography (CPC), with HL in the CHCl3 stationary phase, are mainly due to the slow kinetics of back-extraction of NiL2. The CPC inefficiency as measured by the quantity channel equivalent of a theoretical plate (CETP) bears a direct correlation to the half-lives of the back-extraction reaction. This correlation, for the first time, enables the determination of the interfacial areas generated, and hence the average size of the mobile phase droplets, in the CPC experiments.