In this study, the metabolism of glycogen storage and consumption in mixed cultures under aerobic conditions is described. The experimental results are used to calibrate a metabolic model, which as sole stoichiometric variables has the efficiency of oxidative phosphorylation (delta) and maintenance requirement in units of adenosine triphosphate (m(ATP)). Using the experimental data and values from the literature we show that delta and m(ATP) are strongly coupled and that the values determined for glycogen and poly-beta -hydroxybutyrate (PHB) metabolism are similar. We also demonstrate that storage of glycogen and subsequent growth occur without significant loss of energy, as compared with direct growth on glucose. For kinetic modeling, Monod kinetics is used most commonly in activated sludge models to describe the rate of microbial transformation. Monod kinetics, however, does not provide a good description of the data obtained. Second-order kinetics gives a better description of the rate of glycogen degradation. Formation and consumption of glycogen appears to be much faster than for PHB. (C) 2001 John Wiley & Sons, Inc.