The rate and the yield of the cyan dye formation reaction in single microdroplets dispersed in water were determined for the first time by a laser trapping-spectroscopy-electrochemistry method. The rate-determining step was shown to be the interfacial coupling reaction between electrochemically generated 4-N,N-diethyl-2-methylquinonediimine (QDI) in the water phase and a phenol derivative in an oil droplet. The dye formation efficiency in an individual droplet was also demonstrated to be controlled by the distance between the microdroplet and the electrode. The results were explained in terms of the concentration gradient of QDI between the droplet and the electrode, which was generated by both diffusion and side reactions of QDI in the water phase. Simulation of QDI diffusion in the water phase supported the conclusion as well.