Abstract The resonance frequencies of 13 polygonal vibrations plus 11 axisymmetric vibrations have been observed for a 4.0-mg sessile mercury droplet submerged in 0.10 N KCl solution. When an input voltage with suitable dc and ac components was applied between the mercury droplet and a Ag/AgCl counter electrode, the mercury droplet could be observed under a microscope to oscillate. The mechanism that drives this droplet into oscillation differs from past electrokinetic mechanisms. Basically, the oscillating input voltage causes periodic changes in the interfacial surface tension which, at specific input frequencies, induce resonant vibrations within the droplet by interacting with its expanding and contracting surface area. Because there are two cycles of surface-area variation and two cycles of surface-tension variation per cycle of droplet oscillation, the most effective driving frequency (but not the only driving frequency) is twice the frequency of the droplet oscillation. This work was performed at the Oak Ridge Y-12 Plant, Oak Ridge, Tennessee.