Thermomechanical degradation of polyolefin(s) is carried out in a twin-screw extruder without the use of any additional degradation appts., at 300-500 degrees C and 1-100 bar over a mean dwell time of 0.5-10 min., during which the pressure is (a)periodically altered by 0.5-30 bar. Low molecular wt. polyolefin waxes prepd. by this method are also claimed.