2-Hydroxy-4-n-octyloxybenzophenone oxime (HOBO) was recently synthesized to develop a selective extractant for copper(II). HOBO exhibited a high selectivity for only copper(II) over other base metals such as nickel(II) and cobalt(II) which are typically extracted with LIX series hydroxyoxime reagents in the high pH region from 1 M aqueous ammonium nitrate solution. This is due to the introduction of an alkoxyl group to the phenyl group of hydroxyoximes, which causes a lowering of the dissociation constant of phenol. Copper(II) was extracted as a 1:2 complex with HOBO (=HR) according to the following reaction: Cu2+ + 2HR double left right arrow (CuR2) over bar + 2H (+); K-ex. The extraction equilibrium constant, K-ex = 1.1 x 10(-1), was obtained at 303 K. The extraction rate of copper(II) was measured using a Lewis-type transfer cell at 303 K. The extraction reaction of copper(II) with HOBO is interpreted as an interfacial reaction where the elementary reaction step between the intermediate complex adsorbed at the interface and free extractant in the aqueous phase is rate-determining. The reason this step is rate-determining was inferred.