In the chemical and oil industries, high electrostatic fields have been applied to separate aqueous drops from an oil phase by electrocoalescence. However, very high electrostatic fields can also cause disintegration of aqueous drops which is detrimental to the overall efficiency of the electrocoalescence process. The limit above which an electrostatic field can deform and break up the drops instead of coalescing the drops is of great practical importance. In this paper, the results of an experimental investigation of drop deformation and break-up in different liquid-liquid systems are reported. The onset of drop instability corresponds to the situation when the ratio of the length of the major axis to that of the minor axis of the drop is about 1.9. This corresponds to an electric field strength of between 3.5 x 10(5) and 3.8 x 10(5) V/m for a 1.2 mm diameter aqueous drop, which corresponds to a critical electrostatic Weber number of about 0.49. The value of the critical electric field strength should depend on the initial drop size. The investigation was carried out using a single drop in a rectangular Perspex cell. The drop adopted most commonly a prolate shape. However, if the drop was not in the centre of the electric field, then it could take on different forms altogether. A filament penetrating into the continuous phase could disintegrate and produce one droplet at a time from its tip, and the detached droplets move towards the counter electrode. Movement of a charged drop between a pair of electrodes had also been investigated. To predict the average translational velocity of a charged drop, several aspects need to be considered, such as the shape of the drop deviates from spherical, especially as it contacting and leaving the electrodes. Therefore, the mechanism of drop deformation and break-up under high electric field is complex, and requires more experimental investigation.