The aging properties of asphalts are evaluated using high pressure-gel permeation chromatography (HP-GPC) and Fourier Transform Infrared (FTIR) spectroscopy. The effect of a small amount (1 wt%) of zinc dialkyldithiophosphate (ZDDP) on the chemical and physical properties, and molecular weight distribution of asphalts is studied. The asphalts are aged by means of the Thin Film Oven Test (TFOT) under different conditions. The occurrence of carbonyl groups and the increase in asphalt molecular weight are the aging results in oxidation of base asphalt. ZDDP modified asphalts are resistant to the formation of carbonyl and can keep the asphalt molecular weight nearly unchanged, indicating the improvement of aging resistance of asphalts by the addition of ZDDP. ZDDP as an antioxidant can retard the oxidation of asphalt through the inhibition of peroxides and radical scavenging. (c) 2005 Elsevier Ltd. All rights reserved.