Electra-coalescence technology is commonly used for separation of water from oil. However, the coalescence may not be complete at high field strengths, forming secondary droplets. This process is, hence, undesirable for separation due to the challenging removal of fine droplets. In this study, the effects of non-ionic surfactant (Itmeen 80) on the critical electric field strength (E-crit) of partial coalescence of droplets present at oil-water interface were investigated. Different waveforms and frequencies were employed to gain better understanding of the phenomena. The results showed that the increase in initial droplet radius (R) or surfactant concentration may reduce E(cri)t. Also, E-crit was found to vary linearly with R-0.5 and the slope E-crit/R- 0.5 was proportional to the interfacial tension. Compared to DC electric fields, pulsatile electric field (PEF) can affect E-crit by periodically deforming the droplet. The deformation of droplets was found to be suppressed with elevated frequencies, resulting in an increase in E-crit. Generally, E-crit of DC and bipolar square waveforms were the highest, indicating they were more effective in suppressing formation of secondary droplets. However, E-crit of pulse AC may exceed that of sinusoidal AC and DC at high frequencies. These findings are useful for optimizing electro-coalescence systems. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.