Chemistry in confined volumes, such as aqueous droplets, is different from bulk, continuous water. However, few techniques are available to probe interfacial reactivity in complex, multiphase environments. Here, we demonstrate preferential electroreduction at the oil|water|conductor (three-phase) interface. Electrodeposition of cobalt and nickel results in ringlike structures that can be characterized with tens of nanometers precision in scanning electron microscopy and energy dispersive X-ray spectroscopy. To demonstrate the generalizability of these observations, we show that electroreduction of resazurin to fluorescent resorufin occurs preferentially at the three-phase boundary. The preferential electroreduction does not depend on droplet geometry. These results, grounded in three-phase boundary reactivity, are highly important across all fields of chemistry and biology because they highlight how the interface can change chemistry in unexpected ways.