The interface between two immiscible electrolyte solutions (ITIES) allows for the study of interfacial ion-transfer processes in the absence of interfacial electron transfer. When no common ions exist between the two electrolytes, the interface is polarizable, that is, an electrochemical double layer can be formed under voltage control and ion-transfer rates can be studied as a function of electrochemical driving force. Crucially, the ITIES interface lacks the structural defects present at crystalline solid|liquid interfaces, such as terraces, kinks, and step edges, which are thought to have a disproportionate effect on ion-transfer rates at solid surfaces. In analogy, liquid-mercury|electrolyte interfaces have been used widely to develop our understanding of electron transfer and the structure of the double-layer. Here we report kinetic studies of facilitated proton-transfer (PT) reactions at the ITIES, given the broad importance of PT in water electrolysis, CO2 reduction, and enzyme catalyzed reactions in biology. There have been related efforts to develop theories of proton-coupled electron transfer (PCET) at interfaces, and experiments to understand driving-force dependence of PCET reactions for different catalysts and electrolytes. PT is most often studied indirectly, for example in the context of the hydrogen evolution reaction (HER), where the rate of PT to the electrode surface can be inferred from the overall HER rate under certain experimental conditions. Well-defined hydrogen underpotential deposition (H-UPD) waves have been studied at single-crystal electrodes, which restricts the measured kinetics to a single, self-limited adsorption/desorption step. Yet, in either of these measurements, the distribution and density of structural defects on the surface are not known, and thus the measured rate constant represents an ensemble average of activation barriers across the surface for these distribution of heterogenous sites.