Sulfide (S2-) and nitric oxide (NO) can be formed in anaerobic digester if nitrate and sulfate were presented in the influent streams. This study noted by 4,5-diaminofluorescein diacetate (DAF-2DA) staining and confocal laser scanning microscopy (CLSM) the presence of nitrososulfides (SNO) complex in the S2- + NO-amended methanogenic sludge. Kinetic analysis suggested two-step kinetics involving chemical equilibrium between S2-, NO and SNO as step 1 and the slow conversion from SNO to N2O as step 2. The SNO complex was claimed to have higher inhibition effect than S2- or NO alone on the methangenic activities. Comments were made on the role of SNO complex for the interactions between sulfur, nitrogen and carbon metabolisms in anaerobic digestion. (C) 2013 Elsevier Ltd. All rights reserved.