The ability of flow field-flow fractionation (FlFFF) to characterize the interaction of organic matter with mineral colloids is demonstrated. Colloidal hematite is first prepared and the size distribution is characterized by scanning electron microscopy, FlFFF, and multi-angle light scattering (MALS). Next, the interaction of well-characterized humic and fulvic acids with the colloidal hematite is studied by separating and quantifying the adsorbed and free acids using FlFFF. Both kinetic and thermodynamic information is obtained, including adsorption isotherms. The adsorptive capacity of hematite is greater for humic acid compared to fulvic acid, probably due to the formation of multiple adsorption layers.