This study aims at the question why SND is only working with very low efficiency for granules fed with municipal wastewaster. Therefore three hypotheses were set up: (i) Absence of anoxic zone within the aerobic granules due to deep penetration of oxygen, (ii) low availability of electron donor (carbon source) for denitrification and (iii) low denitrification rate as fraction of large granules is too small. Experiments with batch reactors were performed, with aerobic granular sludge (40-60 % granules) from a column sequencing batch reactor fed with municipal wastewater, to verify the hypotheses. To analyze the influence of a carbon source a distinction of two cases was done. In Case 1 the granules were fed with extra added acetate in a first anaerobic phase. Phosphate,- and glycogenaccumulating bacteria (PAOs and GAOs) stored this acetate intracellular as polyhydroxyalkanoates (PHA) before the aerobic phase started. For Case 2 only particular carbon stored in the matrix of the granules, as extracellular polymeric substances (EPS), was available. Beside that the oxygen concentration was varied between 0.5-2.2 mgO2l-1 and separate tests with extra added nitrate were done to evaluate the denitrification rate. For Case 1 the intracellular stored acetate (PHA) caused a stable simultaneous nitrification and denitrification process whereas for Case 2 unstable situations sometimes caused no denitrifaciton at all. The oxygen concentration was found to be a very important and sensitive parameter. It has a direct impact to the anoxic volume in the granules due to its mass transport limitation by diffusion which is limiting the penetration depth (causing an aerobic surface were nitrification is performed and an anoxic core with denitrifying bacteria). Highest SND ratios were found in Case 1 at an oxygen concentration level of 0.5-1.0 mgO2l-1 and completely taken up and intracellular stored acetate of 100 mgCODl-1. An average SND efficiency of 26.4±13.6 % was reached therefore. Denitrification rates of 1.22±0.16 mgN∙gVSS-1∙h-1 (Case 1) and 0.89±0.05 mgN∙gVSS-1∙h-1 (Case 2) were observed. The fraction of large granules (d>0.63 mm) of only 10 % by weight (gVSS) was relatively low. Therefore it is affirmed that denitrification is low due to this, but not that the denitrification rate is lower than for smaller granules.