Granular activated carbon (GAC) adsorption is recognized as the best available technology for removing synthetic organic contaminants (SOCs) from drinking water. Background dissolved organic matter (DOM), ubiquitous to potable surface and groundwater sources, can significantly impair the performance of GAC systems. In fixed-bed adsorber applications, the DOM mass-transfer zone is more disperse and has a higher velocity than most SOCs; carbon is therefore preloaded with DOM prior to the arrival of the SOC. To design and operate GAC systems effectively, the role of DOM preloading in reducing SOC equilibrium capacity and mass-transfer-controlled rates of adsorption must be understood. There is a need, therefore to identify the components of system heterogeneity. We have investigated the adsorption of humic and fulvic acids by activated carbon and the adsorption of trichloroethylene (TCE) on carbon preloaded with humic substances under different conditions to identify the effects of the DOM molecular size and polydispersity and the chemical composition of the background water.