A new experimental approach for measuring the kinetics of reactions that occur at the interface between two immiscible liquids is described. The interface is created by flowing a feeder liquid through a fine capillary tip at a slow rate, so that droplets form and grow periodically in a second receptor phase in a defined way (analogous to the dropping mercury electrode). The concentration profile of either a product or reactant extending from the interface is probed at a microelectrode positioned directly below the capillary from which the droplet expands. The approach is illustrated through studies of the hydrolysis of triphenylmethyl chloride at the 1,2-dichloroethane/water interface. The reaction rate is determined by measuring the concentration profile for chloride ions potentiometrically at a Ag/AgCl electrode in the aqueous phase adjacent to the interface. By modeling mass transport in the experimental geometry, it is shown that the reaction is first order in triphenylmethyl chloride and occurs heterogeneously with a rate constant of 6.5 x 10(-5) cm s(-1).