Lignin, a natural polymer was used as a filler in low-density polyethylene and polypropylene up to 30 wt%. The influence of lignin blending on processing stability, mechanical properties and light and long-term heat degradation are described for both polymer blend types. It was shown, that there is a different degradation behaviour between polyethylene-lignin and polypropylene-lignin blends. The lignin concentration influenced both the tensile strength and melt flow index. (C) 2000 Elsevier Science Ltd. All rights reserved.