Effects of external carbon sources and COD/NO3-N on nitrite accumulation through denitrification were studied at a temperature of 28 +/- 2.0 degrees C using mixed activated sludge. Nitrite accumulation was observed for each type of carbon source studied. Glucose resulted in the greatest nitrite accumulation and production rate, which were 14.51 +/- 2.41 mg/L and 0.121 +/- 0.013 g N/(g VSS d), respectively. Moreover, a higher COD/NO3-N ratio ranging from 1.0 to 15.0 increased accumulation to the maximum value of 0.34 +/- 0.03 g N/(g VSS d). It was assumed that the competition for electrons between nitrite reductase and nitrate reductase led to different reduction rates and finally caused the accumulation. In addition, it was reasonable to use the pH and ORP as proxies for monitoring the real endpoint of the denitrification process with the addition of carbon sources. (C) 2012 Elsevier Ltd. All rights reserved.