To measure fast reaction processes of the ms order at the interfacial region in the solvent extraction system, we invented a new method using a micrometer-sized two-phase sheath flow system and a laser-induced fluorometry. The reaction between Zn2+ with 5-octyloxymethyl-8-quinolinol (Hocqn) in the 1-butanol/water system was examined. The fluorescence intensity of Zn2+-ocqn complex, which increased along the inner organic phase flow, gave the complexation rate of Zn2+ with Hocqn.