Low-d. polyethylene was modified with allylurea and SiO2 by reactive extrusion and subjected to thermo-oxidn. and to weathering under natural conditions. The thermooxidn. was initiated after a long induction period. Under natural conditions, the oxidn. occurred from the very begining of exposure and yielded acid and ketone groups. Mol. mass values and gel no. of the polymer showed also the crosslinking and crackin of polymer macromols.