Dissolved organic matter (DOM) and its interactions with trace metals play a significant role in the toxicity, transport and bioavailiability of metals in aquatic ecosystems. Work from my laboratory has shown that larger molecular size (MS) DOM fractions had stronger binding ability for copper (II) than lower MS fractions. There was a decrease order of the average molecular weight of organic-bound complexes in stream waters: Cu>Ni>(Co, Zn, Cr)>Pb>Cd for transitional metals, which is consistent with Irving-Williams series, and (Fe, V, Ce)>Th>U>Mo for other metals. This indicates that the distribution of DOM-metal complexes was mainly related to its binding strength. Metals with high strength were more distributed in the larger MS DOM fractions, and metals with low strength were more distributed in the smaller MS fractions. The MS distribution of organic-metal complexes in the XAD-isolated HS was different from that in RO-concentrated samples, indicating that HS isolation procedure affected metal distribution. Several mechanisms for these observations were proposed. This study may have significant implications in the understanding of metal-DOM complexation and fate of metals in aquatic environments.