HPLC-size exclusion chromatography (SEC), coupled with on-line detection of UV absorbance (UVA) and fluorescence, was used to characterize dissolved organic matter (DOM) and its interactions with dissolved metals in a forest stream, a reservoir, and an urban river. Optimization of eluent concentration (20 mM) allowed separation of altogether 11 molecular size (MS) fractions when NaClO(4) was used as an eluent having minimal disturbance with metals. UVA at 254 nm and fulvic-like fluorescence (FFL) at 337/423 (excitation/emission) nm were detected for 'humic' fractions, while protein-like fluorescence (PFL) at 278/353 nm was detected across the whole MS range. At all sites temporal variations in dissolved organic carbon (DOC) concentrations were well explained by changes in UVA intensity of humic fractions, while PFL intensities of a small MS fraction showed a higher correlation with DOC concentrations at the reservoir. Temporal changes in the concentrations of Al, Cr, Li, Ni, and Fe correlated with PFL intensities of small MS fractions, while variations in the concentrations of As, Co, Mn, and Pb were largely explained by UVA and PFL intensities of humic fractions. The results suggest that HPLC-SEC combined with on-line detection of UVA and PFL can be efficiently used in characterizing DOM and its interactions with metals in surface waters.