The objective of the work reported here was to determine whether the ratio of COD/N-ox, has an impact on poly-beta-hydroxybutyrate (PHB) metabolism in activated sludge. Furthermore, it was tested if the ratio influenced the percentage use of organic compounds present in wastewater, for endogenous respiration, oxidation, accumulation and denitrification. Gas flow rate in SBR reactor was controlled by thermal mass flow controller (TMFC). Constant amount of air entering sequencing batch reactor was automatically adjusted to stable set-point 2 mg O-2 L-1. It means that DO concentration in the reactor could change with oxygen uptake. During the filling period and part of the reaction time DO was nearly zero. Feast period of the external substrate availability and famine period of little amount or no external carbon availability were determined. At 23 h of the reaction time, and COD/N-ox, ratio 8, denitrification took place only during feast period. What was interesting, poly- P-hydroxybutyrate degradation was observed in the feast period as well. However, at I I h of the reaction time and COD/N-ox, ratio 37, denitrification occurred in feast and famine period. In the feast period PHB was accumulated and in the famine period was used as the endogenous carbon source. COD consumption to reduce 1 mg N-nitrate was ranging from 1.15 to 6.26 depending on carbon source and increased when exogenous and endogenous carbon were used by activated sludge. The increase in PHB content from 0.25 to 0.43 C-mol/C-mol resulted in a double increase in the amount of nitrogen removed due to denitrification was observed. (c) 2007 Elsevier Ltd. All rights reserved.