A family of silicone oils, of similar density, surface tension, relative permittivity and electrical conductivity but having viscosities in the range 970-58200 mPa s were used to elucidate the role of very high viscosity on the electrically driven flow of liquids. The electrical conductivities of the oils are very low (similar to 10(-9) mS m(-1)) and in effect the behaviour of these dielectric liquids is indicative of the viscosities and electrical conductivities encountered in the processing of concentrated suspensions containing inorganic powders. We show that these oils do not undergo electrostatic atomization in the applied voltage-flow rate regime we have investigated. Nevertheless, electrically forced stable jets are produced from some of the oils and in such instances the jet diameter, size of relics and droplets obtained have been estimated. We show that in the range 10(3)-10(4) mPa s, a stable ball cone jet prevails, however at > 10(4) mPa s the liquids produce micro-threads. (C) 2003 Elsevier Ltd. All rights reserved.