Nanosized activated carbons (NACs) are manufactured particles with sizes <100 nm. NACs obtain negative surface charge in water by deprotonation of functional groups. The aggregation behavior of NACs was consistent with prediction by the DLVO theory. The Hamaker constants derived for AC materials are 2.1–2.7 × 10−20 J in water. NACs may disperse in freshwater but can aggregate in highly saline/acidic water.