Carbon nanotubes (CNTs) ever since their discovery have visualized widespread potential applications in various domains owing to their unique structure and outstanding mechanical and electronic properties. With increasing commercial interests and industrial scale production facilities currently under construction, CNT supply and demand are expected to grow rapidly over the next decade. This high production constitute a serious concern over the health and environmental risks of CNTs once they are released to the environment. Therefore, knowledge of toxic compound adsorption by carbon nanomaterials is critical and useful for the risk assessment of these nanotubes. Adsorption of two synthetic organic compounds (SOCs) by three single-walled carbon nanotubes (SWNTs) was systematically studied under different water quality conditions. For comparison, a microporous activated carbon fiber (ACF10) and a bimodal porous granular activated carbon (HD4000) with similar surface chemistry were also employed in this work. Experimental results demonstrate that the competition between SOCs and NOM uptakes of adsorbents with similar surface affinity to water was not controlled by surface area or pore volume but controlled by their pore size distributions. Both direct competition for adsorption sites and pore blockage by NOM occurred in the adsorption of SOCs. For the microporous ACF10, due to the inaccessibility of the small micropores to NOM macromolecules, pore blockage predominated in the competition, whereas for the bimodal mesoporous HD4000 direct site competition dominates which led to a reduction in SOC uptake and in surface heterogeneity of the adsorbent. Compared with ACF10 and HD4000, the effects of NOM preloading and simultaneous adsorption on SWNTs were much weaker, implying that once released into natural water body, SWNTs will adsorb a considerable amount of toxic SOCs and transfer them to other systems by the transportation of the nanotubes. Compared with the effect of NOM, the effects of ionic strength and pH of the bulk solution on the adsorption of SOCs on SWNTs were insignificant. 2009 © American Water Works Association WQTC Conference Proceedings. All Rights Reserved.