Microelectrochemical measurements at expanding droplets (MEMED) is a new technique for studying the kinetics of reactions that occur spontaneously at the interface between two immiscible liquids. The idea is to create the interface in a well-defined manner by forming a droplet of one (feeder) liquid, by slowly flowing that phase through a tiny (100-mu m-diameter) nozzle submerged in the second (receptor) phase. The interfacial reaction is investigated using an ultramicroelectrode (UME) positioned directly opposite the orifice from which the droplet expands. The UME measures directly the concentration profiles that develop at the expanding droplet due to the interfacial process. Both amperometric and potentiometric electrodes are shown to be suitable boundary layer probes. In the case of amperometric detection, the optimal spatial and temporal response is obtained by deploying the smallest possible electrodes, with characteristic dimensions (radii) of 0.5 mu m or less. Both modes of detection are proven in studies of bromine transfer across an aqueous/1,2-dichloroethane (DCE) interface. These studies, together with investigations of electron transfer between ferrocene in a DCE phase and hexachloroiridium(IV) in an aqueous phase, allow the nature of mass transport in the MEMED configuration to be determined unequivocally. Mass transport is very well-defined in terms of convective diffusion to a symmetrically expanding sphere, although a moving plane model also provides a good description. The well-defined and variable mass-transport regime, coupled with the renewable nature of the interface, makes MEMED a useful technique for investigating liquid/liquid interfacial kinetics. First-order rate constants up to 10(-2) cm s(-1) should be easily measurable.