Pseudomonas stutzeri D6, selectively isolated from activated sludge was used to study NO2- and NH4+ production from denitrification processes. Changes in carbon type, C/N ratio and oxygen concentration significantly influenced the magnitude of NO2- and NH4+ accumulation through denitrification. D6 showed a preference for citrate and acetate, which led to the largest quantity of nitrate reduced and which were exhausted most rapidly, with minimal intermediate products accumulation. It is found that at higher initial organic carbon concentration or for directly metabolic carbon type more complete denitrification could be obtained as a result of increase of the oxygen consumption rate by substrate stimulation. The higher the oxygen concentration in the culture was, the higher the intermediate products concentration became. The experiment showed that NO2- and NH4+ production was only slightly influenced by nitrate concentration. Biological nitrogen removal systems should be optimized to promote complete denitrification to minimize NO2- and NH4+ accumulation. (C) 2011 Elsevier Ltd. All rights reserved.