Low-cost activated carbon fiber composites (ACFCs) were explored as the basis for a system to remove NH 3 from the gas phase. ACFCs were synthesized by chemically activating a phenolic precursor on a glass fiber substrate using ZnCl 2 as catalyst. Additionally, ACFCs were oxidized with concentrated nitric acid at both room temperature and 83 °C to increase the density of surface oxygen groups. Commercially available phenolic-based physically activated carbon fibers were also oxidized under the same condition as a benchmark. Both physical properties and surface chemical properties of these materials were characterized using SEM, BET, elemental analysis, FTIR, XPS, and Boehm titration.Dynamic flowthrough NH 3 adsorption was measured at 500 ppm, at both 0% and 50% relative humidity. Results suggest ACFCs exhibit high surface area, rich oxygen functional groups, with post-treatment further improving surface oxygen content. ACFCs displayed fast sorption kinetics and high NH 3 adsorption capacity up to 50 mg/g. Acidic oxygen groups at the surface are correlated with ammonia adsorption. A new anhydride-based reaction mechanism was identified and is proposed, collectively with carboxylic acid functional groups, as being primarily responsible for retaining NH 3 .