The long-term effects and mechanism of organic matter on ANAMMOX process was comparatively investigated by operating two SBRs. SBR1 was operated under inorganic conditions with an efficient and stable ANAMMOX performance. The autotrophic nitrogen removal performance of SBR2 gradually deteriorated and finally disappeared when influent COD concentration was progressively increased to 800 mg/L during the 220 cycles' operation. After dosing organic matter, the ANAMMOX activity of SBR2 biomass decreased to 1/4 of that of SBR1 sludge. Intensive denitrification and sulfate reduction were observed under the long-term exposure to organic matter. The denitrification and sulfate-reduction activities of SBR2 sludge finally developed to 4 and 16 times of the values of SBR1 biomass, respectively. Thermodynamic and kinetic analyses showed that both denitrification and sulfate reduction were enhanced under the high organic conditions. ANAMMOX bacteria could be eliminated from the SBR system due to preferential metabolization of nitrite by denitrification and, thus causing starvation of ANAMMOX bacteria. (C) 2013 Elsevier B.V. All rights reserved.