A novel approach is reported for obtaining the forward rate law of a rare earth (RE) solvent extraction system. An immiscible water–oil system is studied with a Dy(III) laden aqueous phase and a PC88A laden organic phase with a 1:1 volume ratio, with a total volume of about 0.68 mL. The two phases are poured into a Hele-Shaw cell with a gap width of 1 mm. Hence, a stratified interface forms at the gravitational-perpendicular plane. The Dy(III) concentration boundary layer, as a result of cation exchange, is monitored in real time in the aqueous phase by means of a monochromatic laser-based Mach–Zehnder interferometer. With direct access to the mass concentration at the interface, after spatial integration, an interface mass flux at the initial time is quantitatively resolved for varying Dy(III) and PC88A concentrations. The system is found to be quasi-first order for Dy(III) and quasi-second order for dimeric PC88A. The method can be applied to a broad spectrum of reaction kinetics in systems that are transparent or translucent. The minimum sample volume is 2–3 orders smaller than that required in the conventional method using stirred cells in Lewis or AKUFVE apparatus.