Woodfiber-polypropylene and woodfiber-waste polyethylene composites have been produced by injection molding and by hot pressing the thermoplastic between woodfiber mats. The stress relaxation under constant strain in these composites has been studied at 25, 50, and 80 degrees C. The results have been compared with similar experiments performed on neat thermoplastics. It is interesting to note that the presence of woodfibers as reinforcement in the composites restricts the stress relaxation, but their effectiveness decrease with the increase in ambient temperature. Composites made by hot pressing the woodfiber mat and the thermoplastic are found to exhibit a lesser amount of relaxation than those made by injection molding the same combination. (c) 2006 Wiley Periodicals, Inc.