Activated carbon adsorption is widely used in water treatment for removal of various organic micropollutants; nonetheless, the presence of natural organic matter (NOM) in source water can reduce its efficiency for micropollutant removal. NOM has been shown to compete with target contaminant via different mechanisms: direct competition for available adsorption sites which reduces equilibrium capacity for target compounds, blocking of pore entrance which reduces diffusion rate of the target compounds, and covering of surface sites which may actually enhances diffusion rate of the target compounds. The objective of this dissertation study was to further elucidate the individual competitive effects, to investigate how pore sizes of adsorbents and molecular structure of competing compounds affect competition and to gain a comprehensive understanding of the competitive adsorption. Atrazine, a widely used herbicide in the United States, was used as the trace-level target contaminant and NOM from different source waters as well as NOM surrogates were used as the competing compounds. Powdered activated carbons (PACs) with different pore size distribution (PSD) were used. The PSD was found to have great influence on the pore blockage (PB) effect caused by NOM. The equilibrium capacity of the NOM used in this study was best correlated to the amount of pores of diameter 15-50 ©5, which was also inversely related to the magnitude of the pore blockage effect. Activated carbon that has more surface area in this pore size range had a smaller PB effect on atrazine adsorption kinetics at the same NOM loading. This finding indicated that mesopores are important in realizing adsorption capacity for trace compounds by alleviating the adverse PB effect. The site covering effect was confirmed with additional types of PACs and various competing compounds. More important, the correlation equation that describes the enhanced surface diffusion coefficient for atrazine as a function of the loading of the site-covering compounds was found to be independent of either the PAC type or the competing compound type. The key component was to quantify the competing compound as the equivalent background compound (EBC), which reflected the extent of active sites being covered. iii The site competing effect, the site covering effect and the pore blocking effect were evaluated for five NOM surrogates with different sizes. The smaller molecules were generally more effective in reducing the equilibrium capacity of the target compound. However, for molecules of similar molecular weight, elongated molecules tended to have more equilibrium effect than round molecules. From a kinetic perspective, the enhancement in diffusivity was within one order of magnitude for all five surrogates, while the extent of the PB effect was greatly relying on molecular size that large-sized surrogates caused a much stronger PB effect. Therefore, the overall kinetic effect was dependent on molecular size and the PB effect was usually dominant except for very small molecules. Consistent with the enhanced kinetics associated with pre-adsorbed site-covering competing compounds, atrazine preloading was found to also increase the diffusion coefficient of atrazine, and the extent of enhancement caused by atrazine was greater than that caused by competing compounds. Several explanations were proposed for the difference, which include the micropore filling hypothesis and the artifact associated with the EBC method that was used for site-covering loading quantification.