The removal of humic substances from water is of interest because these compounds form trihalomethanes (THM) and other halogenated organic compounds during chlorination of water supplies. It is possible to maintain THM concentrations at acceptable levels at some locations by carefully controlling chlorination practice and by partially removing THM precursors by coagulation and filtration. At other locations it may be necessary to supplement these measures with adsorption on granular activated carbon (GAC). The purpose of the research reported in this paper was to show how GAC pore size distribution affects its ability to adsorb humic substances, and to show how alum pretreatment of humic substances affect their adsorbability.