Mass transfer processes of ferrocene derivatives (FeCp-X) across a single-nitrobenzene-microdroplet/water interface were studied by laser-trapping and microelectrochemical techniques. Substituent effects of FeCp-X on the mass transfer rate were discussed in terms of the distribution coefficient of FeCp-X between the nitrobenzene and water phases as well as of the adsorption rate of FeCp-X on the droplet/water interface. Interfacial mass transfer of ethylferrocene from water to a droplet was limited by diffusion of the molecule in the water phase, while that of 1-ferrocenylethanol or (N,N-dimethylamino)methylfenocene was governed by both adsorption on the droplet/water interface and diffusion in the water phase.