The binding constant(KDoM) of pyrene onto different molecular weight(Mw) fractions of a dissolved organic matter(DOM) extract from an estuarine sediment was measured via a fluorescence quenching method,and the possible binding mechanisms were discussed.The influence of water chemical parameters on pyrene binding was studied to elucidate the effects of DOM steric conformation.DOM fraction with larger Mw(＞ 14000) showed a greater KDOM(2.02× 105) than that with smaller Mw did,and the KDOMS were 1.16× 105 and 1.13 × 105 for the fractions with Mw of 1000-14000 and ＜1000,respectively.The KDoMS of different Mw DOM fractions were correlated positively with the atomic ratio of C/H and absorbance at 280 nm; while a negative correlation existed between KDOM and (O+C)/N (atomic ratio),and KDOM and O-containing groups.These correlations indicate the positive influence of aromatic structure and the negative effect of polarity on DOM binding capacity.Infrared spectroscopy revealed that specific interactions existed between pyrene and the DOM fraction with larger Mw besides hydrophobic partition,i.e.,NH-π interaction in DOM moiety(1000-14000) and π-π electron donor acceptor(EDA) forces in DOM moiety (＞14000).KDOM varied in a complex pattern with increasing cation concentration and pH,which could be explained by the change in DOM steric conformation.Formed aggregates favored the lipophilic partition of pyrene.However,the accessibility to some interior binding sites became low when the aggregate was too large,leading to a reduced KDOM.