The spontaneous motion of a nitrobenzene droplet on an Au electrode during an Sn electrodeposition has been reported in a previous paper (Chemphyschem, 9, 2302 (2008)). It also mentions that the velocity of the motion increases with a negative shift in the electrode potential. This present work has studied the motive force inducing the droplet motion, which is the imbalance between interfacial tension acting on the opposite side of a nitrobenzene droplet. As explained in the previous study, a high interfacial tension on the front side of the droplet is caused by an occurrence of electrodeposition. From this study, a low interfacial tension on the rear side has been found to attribute to an occurrence of hydrogen evolution reaction on the electrode surface. The rate of hydrogen evolution reaction increases as the electrode potential decreases, and thus the increase in the droplet velocity with a decreasing potential can be ascribed to the increase in the rate of hydrogen evolution reaction. The present work also reports that the velocity also increases by the addition of a salt such as K2SO4 to the solutions. (C) The Author(s) 2015. Published by ECS.