It is also acknowledged that colloids < 0.2 µm suspended in the aqueous phase can be important for determining contaminant availability and mobility. Similarly, dissolved metals can be strongly bound to dissolved organic matter and therefore essentially not bioavailable. We have used flow field-flow fractionation (Fl FFF) and size exclusion chromatography (SEC) coupled to UV-absorbance (UVA) and inductively coupled plasma mass spectrometry (ICP-MS) to study the dissolved organic matter (DOM) and colloidal binding of U and Ni in water extracts of sediments collected from a contaminated area of the Savannah River Site, a U.S. Department of Energy's former nuclear materials production and processing facility, near Aiken, SC, USA. High performance SEC-UVA-ICP-MS was well-suited to the separation of DOM over the molecular weight (MW) range of ~200 – 7000 DA. The ICP-MS element specific data indicated that a significant fraction of U was associated with DOM. Uranium exhibited a bimodal distribution and the other fraction was greater than the exclusion limit for the column and co-eluted with Al. Flow FFF was used to size this fraction as colloidal with an approximate effective spherical diameter of 0.09-0.12 µm. Element specific ICP-MS data confirmed that U and Al were associated with the colloidal phase. High field Fl FFF was also applicable to sizing DOM but resolution was poorer than SEC. The results of this study suggest that ‘dissolved' U at this site is predominantly either complexed by DOM or bound to a colloidal fraction while Ni is predominately present as labile complexes or the free cation and, therefore, potentially bioavailable.