Plasma C black prepd. from methane, ethane, acetylene or ethylene was used, 0.1-1.0 wt.%, to modify cable-grade PE and geomembrane-grade HDPE films. Polymer compns. examd. contained HDPE, LDPE, EVA copolymer, and lrganox 1010 as thermal stabilizer. The C black was found to act both as antioxidant and pigment. Gliding discharge gave the C black of stable and regular structure and high sp. surface (BET), 166-235 m(2)/g. The resulting cables showed longer oxidn. induction times and longer service lifetimes as compared with a com. PE modified with a furnace C black.