Batch-tests were used to determine the potentials of digested sludge to reduce nitrate and nitrite in the presence of five different carbon sources: glucose, glycerol, acetic acid, lactic acid and methanol. Ammonium accumulation was found in glucose and glycerol media. Dissimilatory reduction to ammonium accounted for up to 50% of reduced nitrate and nitrite. The rest were denitrified. In the media containing these carbon substrates volatile fatty acids, particularly acetic acid, were produced and ammonification was higher than denitrification activities only when glucose and glycerol were still present in the media. Ammonium production was higher in nitrite cultures than in nitrate cultures. In the culture media with acetic and lactic acids and methanol, ammonium was not detected. Nitrate/nitrite reduction in acetic and lactic acids media was essentially denitrification activity. Up to 100% of reduced nitrate and nitrite in the culture media with these acids were denitrified at average rates between 27 and 23 mg N-NOx/g MLVSSh, nitrite reduction rate being about 14% lower than total nitrate reduction rate. COD requirements for nitrate and nitrite reductions were generally lower in cultures with acetic and lactic acids than in glucose and glycerol cultures. Methanol culture media showed a very small reduction rate for the N-NOx indicating the absence (or presence in very small quantity) of the bacteria capable of denitrifying with this substrate.