In recent years, a number of investigations have been carried out on space charge formation in the bulk of insulating materials from both the physical and engineering points of view. It has been pointed out that the presence of space charge lowers the breakdown strength.1 It is, therefore, important in the case of utilization of dc power cable to investigate methods for preventing space charge formation. The following methods may be effective: (1) increasing the height of the injection potential barrier at the electrode-insulator interface, (2) trapping the major fraction of the charge in the surface layer of the insulator, and (3) draining charge from the material. In the present study, method (2) has been investigated since it has been reported that carboxyl groups formed by natural oxidation on the polymer surface become trap centres.2