Water droplets dispersed in crude oil have to be separated and this is most commonly done by electrical dehydration. Under high strength electric fields, partial coalescence may occur and leave fine secondary droplets, which reduce the separation efficiency. The critical electric field strength (E-crit) for partial coalescence occurrence depends on several factors. In this paper, the effects of droplet radius, conductivity, interfacial tension, viscosity (changed by adding alkali, surfactant, and polymer respectively) and oil density on E-crit have been studied experimentally. E-crit increases linearly with the inverse of the square root of droplet radius, R-0.5, but the slope E-crit/R-5 (k) can be changed. Increasing surfactant concentration reduces E-crit and the slope k decreases, which indicates reducing interfacial tension promotes partial coalescence. Whereas, adding alkali or polymer improve E-crit and the slope k increases with the increase of its concentration, because of the changes in water conductivity or viscosity. In addition, E-crit is proportional to the product of density difference and oil viscosity. A proposed formula expressing the E-crit, albeit in an empirical way, was given which takes account of the relevant parameters. These results will be of guiding significance to the choice of electrical field strength for electro-dehydration. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.