Environmental colloids are important components of aqueous systems and participate in a host of geochemical processes. Mobile inorganic colloids can accelerate the migration of sorbed contaminants in soils and groundwaters. Organic colloids are important complexers of metals, both in surface and in groundwaters. Field flow fractionation (FFF) is a high-resolution, separation technique that can be used to size fractionate colloids. The elemental composition of colloidal material as a function of particle size can be determined by coupling FFF to inductively coupled plasma-mass spectrometry (ICP-MS). Element distributions can be computed with respect to molecular weight, diffusion coefficient, or hydrodynamic diameter. The results of a series of studies using FFF-ICP-MS are presented, including the characteristics of: soil colloids collected from a number of contaminated sites, organic colloids isolated from the Amazon River basin, and porewater colloids. FFF-ICP-MS results for the porewater colloids are compared to size exclusion chromatography with respect to the size distributions of a number of trace elements.