The effect of selected inorganic anions on the effectiveness of the Fenton advanced oxidative treatment of waters contaminated with methyl t-butyl ether (MTBE) was examined. With respect to the chloride or phosphate ions used, inhibition of oxidation was clearly in evidence, whereas addition of sulfates or perchlorates influenced these rates to a much smaller extent. Anions suppress MTBE decomposition in the following sequence: ClO4- < SO42- < Cl- < H2PO4-. There are very probably several mechanisms affecting oxidation. Complexation reactions with iron are likely to affect the distribution and reactivity of iron species with H2O2. Hydroxyl radicals may also be scavenged from less reactive inorganic radicals. Additionally, in order to explore the degradation mechanism of MTBE by Fenton's reagent in the presence of different salts, four intermediate products of MTBE oxidation were identified: tert-butyl formate (TBF), tert-butyl alcohol (TBA), acetone, and methyl acetate (MA). (C) 2007 Elsevier B.V. All rights reserved.