Experimental results obtained in a batch reactor are presented showing different fast dynamic phenomena. All measured OUR-profiles show a ‘start-up’ phase upon substrate addition. Neither the response time of the DO-electrode, the mixing characteristics in the reactor or the extracellular transport limitations could explain this behaviour. It is hypothesised that intracellular transport and conversion processes are responsible for the transient response. Second, dynamics induced by regulation processes of the macromolecular cell composition could be observed. Finally, the adaptation of the mixed culture population to changed operating conditions is demonstrated. The implications for modelling activated sludge biodegradation with simple structured models identified from such experiments are discussed.