This article investigates the effective liquid-gas surface tension changes of water and 5-90 nm gold nanofluids measured during electrowetting on dielectric experiments. The Young-Laplace equation for sessile droplets in air was solved to fit the experimental droplet shape and determine the effective liquid-gas surface tension at each applied voltage. A good agreement between experimental droplet shapes and the predictions was observed for all the liquids investigated in applied range of 0-30 V. The measured liquid-gas effective surface tensions of water and gold nanofluid decreased with voltage. At a given voltage, the effective liquid-gas surface tension of gold nanofluids initially decreased as the size of gold nanoparticles increased from 5 nm to 50 nm. Then, for 70 nm and 90 nm particle gold nanofluids, the effective liquid-gas surface tension started increasing too. The size of nanoparticles, and the applied voltage have a significant effect on variation of the effective liquid-gas surface tension with variations as much as 93% induced by voltage at a given particle size and 80% induced by particle size at a given voltage. (C) 2016 Published by Elsevier Inc.