Anion-exchange processes have received increased attention in recent years as efficient alternatives for removing disinfection byproduct precursors. In this research the preferential uptake of different dissolved organic matter (DOM) components, and hence the resulting reactivity after treatment, is shown to depend on the initial molecular weight (MW) distribution and the sulfate concentration. MW distribution is important because size-exclusion phenomena can occur in ion-exchange sorption, leading to the preferential uptake of low (ca. 1000 Da) MW species. Sulfate competition can reverse resin preference for low-MW species. DOM components that compete best with sulfate combine ionogenic group affinity and entropy-assisted adsorption. Entropy-assisted sorption, whereby sorption is promoted by the entropy gained from the desolvation of hydrophobic DOM moieties, is shown to be significant for two surface water sources. Entropic contributions are most significant when resin dosages are low and competition between DOM components and between DOM and sulfate are high. DOM components having MW near 1 kDa are sufficiently large to have significant hydrophobic moieties to promote entropy-assisted sorption and sufficiently small to enable access to exchange sites. Total uptake and preferential removal of specific UV absorbance (SUVA), an indicator of DOM reactivity, will thus depend on the initial MW distribution, how SUVA depends on MW, and the sulfate concentration.