The dynamic behavior of the three-phase interface on a platinum electrode during oxygen reduction (ORR) and oxygen evolution reactions (OER) in a potassium hydroxide droplet was studied by a charge-coupled device (CCD) and confocal laser microscopy. Contact angle measurements reveal a spreading interface during ORR, whereas the droplet figure remains unchanged during OER. The microscopy results demonstrated the formation of many fine droplets in the vicinity of the meniscus boundary during ORR, which was attributed to vapor condensation on the surface. Correlation of these observations with electrochemical data and differences in the results between ORR and OER suggest that the motion of the three-phase interface is induced by local pH and temperature gradients in the meniscus, presumably caused by ORR uniformity under the limitation of dissolved oxygen.