The interface voltage appearing when two materials are brought into contact was measured with a high sensitive pressure-wave-propagation measurement setup in various low density and high density polyethylene samples coated either with aluminum or gold electrodes. The samples were then tested under high voltage (40 kV/mm) for 24 h to assess the correlation between interface voltage and charge injection at 22 °C. Aluminum electrodes induce a positive interface voltage toward the insulator favoring electron injection at the cathode while gold electrodes induce a negative interface voltage toward the insulator favoring hole injection at the anode. After the injections due to interface voltage, charges diffuse more easily in low density polyethylene that triggers a subsequent injection due to heterocharges buildup.