Experimental results are presented and discussed that demonstrate the interfacial character of the rate-limiting step (RLS) in the liquid-liquid extraction of Ni(II) by n-hexane solutions of dinonylnaphthalenesulphonic acid (HDNNS) in a non-dispersing stirred reactor. The possibility of a RLS in the bulk aqueous phase was excluded based on the classical criterion of volume-independent transfer rates per unit volume, and a criterion involving dissolution fluxes of the extractant into the aqueous phase. The aqueous diffusion film was excluded as the site to the RLS on the basis of HDNNS partition results. The combination of extractant distribution and dynamic transfer results is shown able to unequivocally identify the site of the RLS for this reactive system.