Activated carbon (AC) is widely used as an adsorbent in water and air purification, but recent studies show that AC can also mediate chemical reactions at ambient temperature, including electron-transfer, redox, free radical, and dehydrohalogenation. Here, we expand the repertoire of such reactions to S(N)2 base hydrolysis of alkyl bromides. Detailed studies were carried out on the quarantine and pre-shipment fumigant, methyl bromide (MeBr), whose removal from fumigation vent streams is sought due to its ozone-depleting potential. Mixed with a 1 M solution of NaOH, ACs are effective adsorbents, but also participate in hydrolysis of MeBr to bromide, methanol, and dimethyl ether. MeBr decay is first order in MeBr and obeys a two-term rate law corresponding to aqueous- and adsorbed-state reactions. The adsorbed-state reaction is, i) unassisted by surface groups, as shown by cycling experiments; ii) 0.83 order in aqueous hydroxide concentration suggesting participation of adsorbed hydroxide; iii) accelerated by pre-adsorption of quaternary ammonium surfactants, which also shift the zeta potential into the positive region; and iv) inhibited by competing inert anions in the order of their chromatographic retention time on ion-exchange resins: Br- < NO3- < ClO4-. The results support an anion exchange mechanism in which hydroxide is attracted to positive sites on the carbon. Normalized to aqueous-phase rate constants, adsorbed-state rate constants for RBr followed the order, R = methyl > ethyl > propyl. (C) 2017 Elsevier B.V. All rights reserved.