Haloacetic acids (HAAs) and trihalomethanes (THMs) are the major disinfection byproducts (DBPs) found in chlorinated drinking water. Nine species of HAAs are commonly found in chlorinated water. Five HAAs are regulated under the Stage 1 Disinfectants and Disinfection Byproducts (D/DBP) rule with a maximum contaminant level set at 60 ug/L. Four species of THMs are also regulated under the D/DBP rule at 80 ug/L. Current methods for DBP control are enhanced coagulation or granular activated carbon (GAC) adsorption for precursor removal. Already formed THMs can be removed using GAC adsorption, but this process is not economically feasible and requires frequent GAC regeneration. The purpose of this study was to investigate the effects of adsorption isotherm test conditions, especially organic matrix and biological activities, on DBP adsorption capacity. The performance of a full scale GAC filter in removing HAAs and THMs was also monitored to verify the adsorption capacities of GAC. Includes 9 references, tables, figures.