The kinetics of production and degradation of poly-β-hydroxybutyrate (PHB) by a mixed activated sludge culture growing on acetate was studied in a sequencing batch reactor (SBR). Occasionally a very high amount of acetate was added to the steady state system in order to obtain high PHB concentrations in the cells (f_PHB). This made it possible to follow PHB production and degradation over a wide range of f_PHB-data (between 0 and 0.8Cmol/Cmol). The results were compared with data available in literature and with equations derived by metabolic modeling. This led to some remarkable observations. For the feast period, the ratio q_PHB^feast/-q_Ac^feast (specific PHB production rate over specific acetate uptake rate) was used to indicate which fraction of the substrate is stored. Experimentally and theoretically it was shown that this ratio has a constant value for dynamically fed systems operated at a sludge retention time (SRT) >2d. This value is 0.6Cmol/Cmol under aerobic conditions and 0.4-0.5Cmol/Cmol under anoxic conditions, irrespective of the specific growth rate of the biomass and the specific acetate uptake rate in the feast period. Degradation of internal stored PHB could be described with a first order degradation rate with respect to the PHB content of the cells. Degradation of PHB appeared to be independent of the type of electron acceptor present in the system and independent of the SRT of the system. The kinetic descriptions can be used to predict PHB production and consumption in general in dynamic fed wastewater treatment systems, and they provide some trends for modeling purposes.