The dependence of the direct electrical breakdown voltage of degassed transformer oil and of gas-saturated oil on gap length for a point-sphere electrode system has been studied for the two polarities of the point. The effects of anodization of the sphere electrode on the breakdown voltage against gap characteristics have also been examined. The critical gap length at which the breakdown voltage is the same for both polarities of the point has been found to depend on the nature of the insulating film on the sphere electrode and on the type of gas dissolved in the oil. The discussion of the results is based on the propagation of a corona discharge from the point electrode and the possibility of formation of a back streamer from the sphere electrode.