Phosphate accumulating organisms (PAO) and glycogen accumulating organisms (GAO) play an important role in aerobic granular biofilm systems used for wastewater treatment. Both are slow growing organisms which can take up and store carbon under anaerobic conditions. The growth of PAO over GAO is preferred because of their capacity to remove phosphate. As some PAOs and GAOs can utilize nitrite or nitrate as electron acceptor in absence of oxygen, they act also as denitrifiers (i.e. DPAO/DGAO). The presence of active DPAOs in a treatment system is desirable because the same carbon source is used for dephosphatation and denitrification. The aim of this study was to investigate and to compare the denitrification potential of aerobic granular biofilms having already a good phosphate removal activity, and that were cultivated with either propionate or acetate as model contaminats. The results show that both biofilms had considerably higher specific uptake rates for nitrite than for nitrate (3.26 times for acetate and 4.79 times for propionate). These results agree with a study (Martin et al., 2006) which showed, that some DPAO have no nitrate reductase genes and can therefore only reduce nitrite but not nitrate. Therefore, DGAO were likely responsible for nitrate and DPAO together with DGAO for nitrite removal. This assumption is supported by the observation that the nitrite uptake rate decreased in the absence of phosphate, whereas the nitrate uptake rate was not influenced by phosphate presence or absence. Furthermore we observed a 27% higher specific nitrite uptake rate in the propionate reactor than in the acetate reactor. This supports the hypothesis that granules fed with propionate may favor the development of PAO with nitrite reducing capacity. We conclude that (i) to enhance the denitrification and create a link to the P-removal, nitrite oxidation to nitrate has to be inhibited, and that (ii) propionate leads to more robust DPAO populations than acetate. In addition to the biochemical results presented in this abstract, microbial community analysis of granules will be done to complete the results.