The thermal stability and the mechanical properties of modified and unmodified glass-reinforced polypropylene during aging in air (135 degrees C) and antifreeze (115 degrees C) with exposure up to 80 days was investigated in this study. Aging leads to progressive phase separation. On the contrary, the original modified composite is characterized by a dense structure; aging leads to the structure compacting and pore size decreases. Unmodified composite displays a drastic failure of the mechanical properties after 10 days of heating. (C) 2000 John & Wiley & Sons, Inc.