In order to analyse the pre-breakdown phenomena in insulating oil, a hot tungsten filament emitting an abundance of electrons in a vacuum was employed as cathode. The gap between the electrodes was therefore only partly occupied by the liquid. Currents measured with various voltages and depths of oil were explained on the basis of space-charge limitation in the oil, and the mobility of the negative ion in transformer oil formed by the attachment of electrons to oil molecules was found to be 1 × 10^-4 cm/s per V/cm.